Comparison of the Hadza hunter-gatherers & non-human primates: The sexual division of labour likely co-evolved with increased sex differences in spatial behaviour and landscape use

Gendered movement ecology and landscape use in Hadza hunter-gatherers. Brian M. Wood, Jacob A. Harris, David A. Raichlen, Herman Pontzer, Katherine Sayre, Amelia Sancilio, Colette Berbesque, Alyssa N. Crittenden, Audax Mabulla, Richard McElreath, Elizabeth Cashdan & James Holland Jones. Nature Human Behaviour, Jan 4 2021. https://www.nature.com/articles/s41562-020-01002-7

Abstract: Understanding how gendered economic roles structure space use is critical to evolutionary models of foraging behaviour, social organization and cognition. Here, we examine hunter-gatherer spatial behaviour on a very large scale, using GPS devices worn by Hadza foragers to record 2,078 person-days of movement. Theory in movement ecology suggests that the density and mobility of targeted foods should predict spatial behaviour and that strong gender differences should arise in a hunter-gatherer context. As predicted, we find that men walked further per day, explored more land, followed more sinuous paths and were more likely to be alone. These data are consistent with the ecology of male- and female-targeted foods and suggest that male landscape use is more navigationally challenging in this hunter-gatherer context. Comparisons of Hadza space use with space use data available for non-human primates suggest that the sexual division of labour likely co-evolved with increased sex differences in spatial behaviour and landscape use.

Discussion

Our study demonstrates that substantial gender differences in spatial behaviour were present across the life course in this hunter-gatherer society. Among the Hadza, men travelled further, visited more areas of the landscape, followed more sinuous routes and were more solitary while foraging. These patterns are consistent with the gendered ecology of hunting and gathering, in which men search for rarer, more energy-dense and more mobile resources than women23,26,27,46. These metrics of spatial behaviour also suggest that male-typical travel is more navigationally challenging. An earlier study3 showed that Hadza men scored higher on average than Hadza women in measures of spatial cognition and performance, a result that is consistent with the gender difference in spatial behaviour seen here. Gender differences in Hadza spatial behaviour emerge early; by the age of 6 years, boys are travelling slightly further on a daily basis (Cohen’s d = 0.1, 95% CI = 0.02–0.18; Supplementary Table 5). This means that the developmental environments that boys and girls experience are spatially distinct from an early age, and they continue to be so across the life course. It is important to note the following limitation of this study: as with most studies of gender differences, there is no way for our study to disentangle whether gender differences in spatial cognition or performance emerge solely from differences in developmental environments or are also impacted by innate physiological differences. Future work that examines changes in Hadza spatial behaviour and cognition in those areas where the traditional hunting and gathering economy has declined may provide useful insights into the consequences of different developmental environments.

The emergence of a gender difference in Hadza spatial behaviour around 6 years of age is broadly similar in timing to meta-analysis results7 showing that gender differences in spatial cognition emerge in the years 7–14 in Western samples. A study of children in the UK reported that gender differences in both exploratory behaviour and spatial cognition are detectable in the age range of 6 to 11 years47.

The Hadza’s highly mobile lifestyle is readily apparent in our data. Adult Hadza women walked an average of 7.6 km per day and logged 10,888 steps, while men walked 12.9 km and logged 18,476 steps. For comparison, a GPS study in Pretoria, South Africa48 found that urban women and men walked much less (2.9 and 3.9 km per day, respectively). Mbendjele women in the Republic of Congo, who practise a mixed farming and foraging lifestyle, also appear to have walked less than Hadza women (median of 4.35 km per day), but the Mbendjele study18 differed from ours in recording only travel out of camp, for 5.2 hours per day on average, while our study records all travel, for an average of 11.49 hours per day. Hadza women’s step counts were about twice as high as average smartphone-using subjects enrolled in a 111-nation study45, and Hadza men’s about three times higher than men in this global sample from middle- and high-income countries. Other rural, non-industrial societies may walk even more than the Hadza: a study of an Old Order Amish49 community in Ontario, Canada reported that adult women and men logged 14,196 and 18,425 steps per day, respectively.

It is interesting that we did not find clear statistical evidence that young children limited how far their mothers walked each day. Perhaps because Hadza plant foods are so abundant, there was little foraging incentive for a woman without young children to travel faster or further than another woman in her party carrying a small child. It seems equally plausible that women with small children are incentivized to keep up with the group. A recent study among the Twe of Namibia also found no difference in daily distances travelled between nursing and non-nursing reproductively aged women19. A limitation of this study is that subadults were not sampled as frequently as adults, for reasons discussed in Methods. In a future study, we hope to examine more fully the coupled patterns of movement by women and their children.

A limitation of our study is that we identified two features of socioecology that are likely to limit Hadza women’s travel relative to men’s: the fact that plant foods are immobile and found in much greater abundance than men’s foods, and the fact that women appear more fearful of encountering Datoga pastoralists while foraging out of camp. Our sense, based on many years of ethnographic research, is that the Datoga threat is kept in check by armed Hadza men and older boys who accompany parties of foraging women, and that the gender differences in spatial behaviour we identify here are by and large owing to the foraging ecology. A future study that examines movement patterns in areas with more or less Datoga presence should shed light on this issue.

The strong gender differences in space use observed here have implications for disease ecology and health measures. Our data show that men travelled more expansively (Fig. 3, Table 1 and Fig. 5), so one could reasonably assume that they were exposed to more diverse pathogens. Men are also more likely to be exposed to zoonotic pathogens because of interacting with and butchering animals. Sex differences in host immune responses are reported for many species, including humans50. Spatial data like those presented here should factor into causal modelling of such differences.

As evolutionary anthropologists, we are compelled to ask how these data can inform reconstructions of the past. Patterns of behaviour observed in diverse samples of contemporary hunter-gatherers are more likely to have been a part of our species’ evolutionary history than are patterns of daily life observed among university students or citizens of rich nation-states, who today make up most research subjects in social science51. The broad gender differences in spatial behaviour that we observed among the Hadza are likely to have been a regular feature of hunter-gatherer societies in the past and to also be present today where gendered foraging persists. Cross-cultural analyses show that hunting is a male-typical activity, and that when women do hunt, they tend to focus on smaller and less mobile prey46,52,53. The male hunting specialization is not a function of contemporary gender roles per se, as it is also consistent with signs of lower limb morphology and wear seen in prehistoric skeletal samples54. Interestingly, among non-human primates that occasionally hunt vertebrates, including chimpanzees, baboons and capuchins, males hunt more frequently than females55,56,57.

Over the last 2.5 million years, diets in the genus Homo shifted to include more hunting and pursuit of mobile foods53,55,56,58. These shifts undoubtedly increased home range sizes and are likely to have increased sex differences in spatial behaviour. Among living apes, sex differences in ranging are comparatively modest and reflect sex differences in reproductive ecology and territory defence. Male orangutans maintain larger home ranges than females despite similar diets, as males apparently seek increased access to females59. In chimpanzees, males travel an average of ~20% farther than females each day (3.6 versus 3.0 km per day), reflecting males’ larger foraging group sizes and territory defence60. Mountain gorilla males, whether solitary or in groups, have similar daily travel distances to females61. Studies of non-human primate ranging using GPS devices are rare, but GPS data from a recent study of olive baboons62 show little to no evidence for a sex difference. By contrast, the day ranges of Hazda men are 14.3 km per day at age 25 years and are 62% higher than those of Hazda women (Supplementary Table 3). An increase in gender-differentiated foraging over the last 2.5 million years may have increased sex differences in spatial cognition. More studies of spatial behaviour and cognition among non-human primates—especially apes—would be very helpful for constructing maximally plausible models of hominin cognitive evolution. At present, this interspecies analysis is limited to only a few studies in which precise spatial measures have been collected.

Our study has demonstrated a high degree of gender segregation in Hadza landscape use. It is worth remembering that this segregation out of camp is bookended with intense sociality and co-operation in camp among all co-residents24. Like other hunter-gatherer societies, the Hadza practise central place foraging, which anchors people’s movements to a camp and socially embeds them into a co-operative network of neighbours. Over evolutionary time, central place foraging and language would have fundamentally changed our genus’s spatial strategies. Among much else, it would have permitted early humans to form much vaster and dynamic mental models of landscapes by incorporating others’ spatial knowledge and their own experiences. It is routine for Hadza to exchange information at the end of the day, detailing their daily travels and experiences, and men and women often relay to one another any promising signs of plant or animal life they encounter. These derived features of human sociality mean that our spatial strategies are likely to differ strongly from those of other primates, above and beyond the role played by gender-differentiated foraging.

Research in movement ecology is examining space use with sophisticated methods and theory from diverse fields18,62,63,64,65,66, and these tools are increasingly being used to examine human behaviour. Such studies, carried out across diverse cultural contexts, will allow researchers to identify regular features of human spatial behaviour and shed light on spatial adaptations to varying climatic, economic and epidemiological conditions.

Sadness pervades affective responses to death; words chosen to represent perceptions of others’ feelings towards death suggest that we perceived others as feeling more negative about death than they we do ourselves

Miller-Lewis LR, Lewis TW, Tieman J, Rawlings D, Parker D, Sanderson CR (2021) Words describing feelings about death: A comparison of sentiment for self and others and changes over time. PLoS ONE 16(1): e0242848, Jan 6 2021. https://doi.org/10.1371/journal.pone.0242848

Rolf Degen's take: https://twitter.com/DegenRolf/status/1347045740305133572

Abstract: Understanding public attitudes towards death is needed to inform health policies to foster community death awareness and preparedness. Linguistic sentiment analysis of how people describe their feelings about death can add to knowledge gained from traditional self-reports. This study provided the first description of emotive attitudes expressed towards death utilising textual sentiment analysis for the dimensions of valence, arousal and dominance. A linguistic lexicon of sentiment norms was applied to activities conducted in an online course for the general-public designed to generate discussion about death. We analysed the sentiment of words people chose to describe feelings about death, for themselves, for perceptions of the feelings of ‘others’, and for longitudinal changes over the time-period of exposure to a course about death (n = 1491). The results demonstrated that sadness pervades affective responses to death, and that inevitability, peace, and fear were also frequent reactions. However, words chosen to represent perceptions of others’ feelings towards death suggested that participants perceived others as feeling more negative about death than they do themselves. Analysis of valence, arousal and dominance dimensions of sentiment pre-to-post course participation demonstrated that participants chose significantly happier (more positive) valence words, less arousing (calmer) words, and more dominant (in-control) words to express their feelings about death by the course end. This suggests that the course may have been helpful in participants becoming more emotionally accepting in their feelings and attitude towards death. Furthermore, the change over time appeared greater for younger participants, who showed more increase in the dominance (power/control) and pleasantness (valence) in words chosen at course completion. Sentiment analysis of words to describe death usefully extended our understanding of community death attitudes and emotions. Future application of sentiment analysis to other related areas of health policy interest such as attitudes towards Advance Care Planning and palliative care may prove fruitful.

Discussion

Summary of findings

Understanding public attitudes and feelings towards death is needed in order to inform health policies [3]. This study provided the first description of emotive attitudes of people asked to consider death utilising sentiment analysis. Applying multidisciplinary methodologies to a novel activity gave a deeper understanding of affective death attitudes in a community sample. We analysed the emotional sentiment of words people chose to describe feelings about death, for themselves, for perceptions of others, and for changes over the time-period during exposure to a course about death. The results demonstrated that text-sentiment analyses can provide a meaningful approach to death attitude research, consistent with previous linguistic investigations [20–22, 29]. ‘Sad’ was a word that was prevalent throughout, regardless of whether referring to feelings for oneself or for others, or feelings captured at the beginning or ending of a course about death and dying. Thus ‘sad’ feelings were universally linked to death. Nonetheless, the MOOC participants commonly chose words to express their perception of other’s feelings towards death that were considerably more emotionally negative (‘fear’, ‘scary’, ‘loss’) than the words chosen to express their own feelings towards death (‘inevitable’, ‘peace’, ‘natural’). When we moved beyond the self-report of words themselves and applied numeric sentiment scores to the words chosen, we found that the sentiment expressed to represent one’s own perspective was significantly more positive compared to those representing the perceived perspective of others. The words chosen to represent the feelings of others indicated sentiment that was more unhappy/unpleasant, more arousing/excitable, and more submissive/dominated by external forces. This aligns with findings from other studies [5, 59, 60]. Our findings leave us with the question of why the MOOC participants think that others feel so differently about death than they do themselves? Part of the explanation is likely to lay in the self-selected nature of our sample of people who chose to participate in a course about death, a considerable proportion of whom identified as health professionals and may have seen themselves as more informed and conditioned to death. But, the sentiment of the words chosen to represent the perceived feelings of others does demonstrate the assumptions people make about other peoples’ negative reactions regarding death. It is possible that these assumptions have implications for the willingness of people to start conversations about death and dying with the people around them. If we perceive that others will become distressed by bringing up the topic of death, are we less likely to attempt raising the topic? Does this avoidance then leave important things unsaid?

When we compared the types of words participants chose to describe their personal feelings about death at the beginning and the end of the MOOC, we found that the most frequently mentioned words were quite similar at both time points (e.g., inevitable, peaceful, peace, natural, sad), but that mentions of more negative words like ‘sad’ reduced considerably in frequency over time, along with a corresponding increase in mentions of words related to acceptance and comfort. The specific words chosen by individuals at each time point showed more changes over time than stability. When the sentiment of the words chosen to express personal feelings about death at both time-points was analysed, we found that participants chose happier valence words, less arousing words, and more dominant words at the end of the course about death than they did at the beginning. This finding suggests that participating in an online course about death and dying may have had a positive effect on the type of language people chose to express their feelings about death, with the emotional sentiment of the language used by the end of the course subsequently becoming more pleasant/positive, calmer, and more internally controlled. Given that the sentiment of words deals with emotion in relation to an issue, this suggests that over time the course may has assisted participants in becoming more emotionally accepting in their feelings and attitude towards death. It is also important to note therefore that spending time thinking and learning about death over a five-week period did not appear to have a negative emotional impact on course participants. This finding is consistent with our findings from research with our 2016 cohort using formal scale measures [6] and evaluation questionnaires [5] geared more towards behavioural indicators. The findings of the present study indicate that participation in the MOOC potentially had a positive emotional influence on the participants, in addition to the behavioural and cognitive influence demonstrated in earlier studies [5, 6]. This offers valuable triangulating evidence validating the potentially beneficial effects of participating in an online course about death.

Our multivariate findings demonstrated that the positive changes in sentiment occurred similarly for participants regardless of education, occupation, and location. However, an interesting influence of age was found on the rate of change in valence and dominance sentiment scores from the beginning to the end of the course. From the outset, younger participants scored lower on death sentiment valence and dominance, and compared to older participants, younger participants experienced greater change during course participation on the sentiment of the language chosen to express personal feelings about death, with greater increases in the pleasantness (valence) and dominance (power and control) in the words they chose at the end of the course. This may be in part due to younger participants having lower valence and dominance scores at baseline, meaning there was more opportunity for improvement in their scores by the end of the course. For older participants, the influence of a course discussing death is perhaps less due to having more personal exposure and proximity to death in their life and thus greater mortality salience. Therefore, it’s possible that participation in an online course about death may be especially beneficial for younger people, potentially assisting them to become less emotionally negative and more accepting of death and its inevitability.

Implications

The results from this study provide further validation of the emotive nature of death and dying. It provided quantification of the strength and direction of affective responses to death and the differential perceptions of death attitudes participants applied to others in the community. Word sentiment analysis was a useful adjunct to traditional self-reports, and is arguably less invasive.

The words and labels used in clinical encounters can have differing effects upon patients and clinicians. For example, Tayler and Ogden [81] reported the tendency of doctors to use euphemisms instead of the direct term ‘heart failure’ due to the dilemma of the latter causing more negative emotional reactions in patients. On the other hand, a study of more benign conditions (gastroenteritis/tonsillitis) found the use of clinical language over lay language can validate the patient’s sick role and can increase confidence in the doctor [82]. The use of euphemisms to avoid the word death is reportedly common, and accompanied by the potential for misunderstandings [20, 22, 58]. Thus, words are not neutral. They can have an impact on patients and people–they can bring people with us or alienate them, they can be interpreted differently by people, they can be delivered in personal or impersonal ways, and they can sway a person’s actions. There may be implications worth exploring for clinical settings, particularly for awareness of the choice of words verbalised in clinical encounters related to the end-of-life. Understanding the sentiment of words provides valuable insight into the emotional connotations tied to words we use, and could be useful for guiding clinical conversations in palliative care [20, 22].

The knowledge gained from this study about community death attitudes could potentially inform the creation of dialogue and messaging used in public health campaigns about the end-of-life preparedness. The use of higher valence and dominance words might bolster the persuasiveness of campaigns in a way that potentially leads to more effective emotional engagement and behavioural activation in target populations [37, 83].

Strengths, limitations, and future directions

Whilst this study was based on a large sample, several potential limitations require consideration. Like most MOOC samples, our sample is unlikely to be representative of the general population [62–64]. Participants were a self-selected sample of people in the community who chose to enrol in a course about death, and therefore may have been more likely to feel comfortable with the topic of death from the outset than people in the general population. Our sample also included a considerable proportion of people who identified as health professionals, which may impact on death attitudes given their greater opportunity for exposure and possibility of more desensitised death-related emotions. The sample was also comprised of a vast majority of females, which meant that gender comparisons could not be meaningfully undertaken. Furthermore, the results of the present study may be somewhat less representative of younger people and those residing outside Australia, because enrolees who subsequently went on to participate in the MOOC activities were slightly older and more likely to be located in Australia than those who enrolled but did not participate. Replication of the study with a representative community sample would be a valuable avenue for future research, to garner a stronger understanding of socio-demographic variations in the sentiment of words about death. Comparing death word sentiment for various age cohorts in the community with differing death exposure experiences is particularly needed, as well as direct dyad comparisons of self-other death attitude perceptions. It would also be informative to examine how respondents’ death word sentiment relates to their self-reported fear of death and other emotive death attitudes rated on standardised instruments. This could enable a broader insight into various aspects people associate with death [7].

A strength of this study was the inclusion of a longitudinal component that observed changes over time in the sentiment of words chosen to express feelings about death. However, course attrition meant that slightly less than half of participants that provided words at baseline also provided data at the end of the course. It is possible that those participants who were less inclined to complete the Time 2 activity were also less likely to have experienced a positive emotional response to the course. Nonetheless sensitivity analyses using alternative approaches (Multiple Imputation) to account for the missing data obtained similar results and conclusions to those using the complete-case sample. In the final week of the course when participants were asked to provide three death words again, it was possible for them to go back and look at their previous word choices from the beginning of the course. It is not clear how many participants went back and checked their first response, versus those who followed instructions to simply go with their instinctual response. It is not known what effect any comparative check by participants may have had on the words they chose to report at Time 2.

Of further importance is that our longitudinal assessment of change over time did not include a comparison group of people who were not exposed to the MOOC. Without this comparison group, it is impossible to rule out alternative explanations for the reasons word sentiment scores changed from the start to the end of the course. Other factors occurring within lives of participants while completing the MOOC may have caused changes on death sentiment over time that were not related to MOOC completion. Future research including a control group is warranted.

In this study, we applied sentiment analysis to three specifically chosen content words rather than to a written sentence, or a more generalised analysis of written text. However, it is possible that by giving participants explicit instructions we obtained more expressive emotional content words than we would have if a sentence was requested, given that typically only a small proportion of words used are classified as emotive [14]. The results may have differed if the activity used a sentence instead, but it may have reduced the emotive data elements provided, being more likely to have been limited to one core theme/lemma. Alternatively, utilising a much larger textual response may have provided a deeper understanding of emotions, as there is evidence that non-content function words such as prepositions and pronouns can also convey affective content [84, 85]. It would be worthwhile to explore the use of these different methodologies in future research, such as examining the effect of the online course over time by comparing the sentiment expressed in larger bodies of general comments/posts made at the beginning and at the end of the course.

Another potential limitation is that Warriner and colleagues’ [37] lemma database is based on rating words on an ordinal 1-to-9 scale, rather than a continuous interval scale. There may not be a uniform difference reflected between a rating of 1-to-2 as there is for 8-to-9. Very recently, new slide-rating methods for valence scores have been tested, which may provide a fruitful direction for future studies to utilise, if norms are developed [86].

The current study focussed on words used to describe feelings about death and dying, and has provided valuable insights into the way people feel about this issue. There is considerable potential for applying sentiment analysis to other related constructs, such as words chosen to describe feelings about Palliative Care, Advance Care Planning and Voluntary Assisted Dying. Such future research may help us better understand the nuances in community attitudes towards these important social policy issues. The possibilities of sentiment analysis as a methodology to detect emotional states of respondents also deserves future research attention. Previous studies have successfully used sentiment analysis to detect mental health conditions and level of threat in social media statements [17, 32]. Future research could pursue word sentiment as a method to detect emotional distress in health settings (e.g., carer bereavement risk), which could activate early intervention efforts.

Born to Shop? Past research indicating that deal-prone consumers are likely to have one or more deal-prone parents has suggested that there may be genetic factors involved in the appeal of retail sales promotions

Born to Shop? A Genetic Component of Deal Proneness. Robert M. Schindler, Vishal Lala, and Jeanette E. Taylor. Journal of the Association for Consumer Research, Jan 5 2021. https://www.journals.uchicago.edu/doi/abs/10.1086/710244

Rolf Degen's take: https://twitter.com/DegenRolf/status/1346844597289410561

Abstract: Past research indicating that deal-prone consumers are likely to have one or more deal-prone parents has suggested that there may be genetic factors involved in the appeal of retail sales promotions. We report the results of a study of monozygotic and dizygotic same-gender twins that indicate a role of hereditary factors in the degree of consumer interest for at least some types of promotional deals.

Maybe rodents regulate a ratio of protein to dietary carbohydrates in order to achieve metabolic benefits (reduced insulin levels, improved blood glucose control, and, in the long term, reduced weight & fat gain)

What does self‐selection of dietary proteins in rats tell us about protein requirements and body weight control? Patrick C. Even  Joséphine Gehring  Daniel Tomé. Obesity Reviews, January 5 2021. https://doi.org/10.1111/obr.13194

Rolf Degen's take: https://twitter.com/DegenRolf/status/1346836414521081859

Summary: Omnivores are able to correctly select adequate amounts of macronutrients from natural foods as well as purified macronutrients. In the rat model, the selected protein levels are often well above the requirements estimated from the nitrogen balance. These high intake levels were initially interpreted as reflecting poor control of protein intake, but the selected levels were later found to be precisely controlled for changes in dietary protein quality and adjusted for cold, exercise, pregnancy, lactation, age, etc. and therefore met physiological requirements. Several authors have also suggested that instead of a given level of protein intake, rodents regulate a ratio of protein to dietary carbohydrates in order to achieve metabolic benefits such as reduced insulin levels, improved blood glucose control, and, in the long term, reduced weight and fat gain. The objective of this review was to analyze the most significant results of studies carried out on rats and mice since the beginning of the 20th century, to consider what these results can bring us to interpret the current causes of the obesity pandemic and to anticipate the possible consequences of policies aimed at reducing the contribution of animal proteins in the human diet.

Chimpanzees: Adolescent males mated with nulliparous females and reproduced primarily with these first-time mothers, who are not preferred as mating partners by older males

The development of affiliative and coercive reproductive tactics in male chimpanzees. Rachna B. Reddy, Kevin E. Langergraber, Aaron A. Sandel, Linda Vigilant and John C. Mitani. Proceedings of the Royal Society B: Biological Sciences, January 6 2021. https://doi.org/10.1098/rspb.2020.2679

Rolf Degen's take: https://twitter.com/DegenRolf/status/1346820516317949954

Abstract: Like many animals, adult male chimpanzees often compete for a limited number of mates. They fight other males as they strive for status that confers reproductive benefits and use aggression to coerce females to mate with them. Nevertheless, small-bodied, socially immature adolescent male chimpanzees, who cannot compete with older males for status nor intimidate females, father offspring. We investigated how they do so through a study of adolescent and young adult males at Ngogo in Kibale National Park, Uganda. Adolescent males mated with nulliparous females and reproduced primarily with these first-time mothers, who are not preferred as mating partners by older males. Two other factors, affiliation and aggression, also influenced mating success. Specifically, the strength of affiliative bonds that males formed with females and the amount of aggression males directed toward females predicted male mating success. The effect of male aggression toward females on mating success increased as males aged, especially when they directed it toward females with whom they shared affiliative bonds. These results mirror sexual coercion in humans, which occurs most often between males and females involved in close, affiliative relationships.

4. Discussion

The results presented here suggest that adolescent male chimpanzees, who cannot effectively compete with older males nor sexually coerce adult females, employ at least two behavioural tactics to mate and reproduce. First, as reported in previous research, adolescent males appear to target adolescent, nulliparous females as mating partners; they mate with nulliparous females frequently and father their first offspring more often in adolescence than they do in adulthood [42,48]. Our findings also corroborate past research indicating that nulliparous female chimpanzees are less preferred as mating partners than are parous females. Specifically, as male chimpanzees transition from adolescence to adulthood and rise in dominance rank, they show less sexual interest in nulliparous females and target them for aggression infrequently [47]. High-ranking males also rarely father the first offspring of these females [33–37,42]. Second, mating success for adolescent and young adult males was predicted by the strength of affiliative bonds that males formed with females. Male aggression, by contrast, had a relatively weak relationship with mating success, but one that strengthened as males grew older and increasingly dominant to females.

These findings increase our understanding of the nature of sexual coercion in chimpanzees. We have recently shown that adolescent and young adult males selectively direct aggression toward females with whom they form strong affiliative bonds [49]. Here, we demonstrate that aggression has a reduced effect on mating success outside of these bonds for young, adolescent males who are not yet physically mature and unlikely to dominate females [49,54,71]. Instead, mating success increases when an older adolescent or young adult male directs aggression to a female with whom he frequently affiliates and can dominate.

These results complement prior research that indicates aggression, mating and reproduction are linked in chimpanzees [28,30,31] and clarify the role that affiliation plays in creating those linkages. Specifically, sexual coercion is more effective when adolescent and young adult males have affiliative bonds with the females they attack. One reason may be that females suffer higher costs if they refuse to mate with males with whom they frequently affiliate compared to males with whom they rarely affiliate [12]. The nature of these costs remains to be explored. Nor is it clear whether and how affiliative relationships with males benefit female chimpanzees. It is important to note that our findings are consistent with patterns of intersexual aggression in other species where males are highly aggressive to females with whom they share bonds. This includes hamadryas baboons (Papio hamadryas), where females live in one-male groups, and most social activity is directed by the single males in these groups, i.e. ‘leader males’. After being attacked by their leader male, hamadryas females appear fearful and follow him even more closely than they had previously [72]. In our own species too, many women are subject to frequent sexual coercion by their male partners, but often remain in such relationships for reasons that vary widely [73].

Scant data exist about the proximate psychological mechanisms that underlie male aggression and female compliance in chimpanzees. However, investigating these proximate mechanisms may provide information about how bonds that affect paternity in chimpanzees might lead to a human-like social system [74]. One interpretation consistent with our preliminary observations is that male aggression toward their social partners is motivated by sexual possessiveness (e.g. [73]), and that females have a psychologically distinct experience when attacked by a male with whom they have an affiliative bond. For example, adolescent and young adult males make direct attacks on male peers infrequently, but when they do so, it is when another male mates or attempts to mate with one of their female social partners [49]. Anecdotally, when female chimpanzees received aggression from an adult male who did not have a strong affiliative bond with them, they often just screamed and ran away. Females receiving similar aggression from males with whom they shared strong affiliative relationships, however, react in an entirely different way. When attacked, these females remain in place, lunge toward their male partners while clutching their arms, rocking back and forth, and screaming repeatedly until making choking sounds.

Our study has several limitations. First, we cannot evaluate the relative impacts of affiliation and aggression on adolescent male paternity success. Only seven males in this study have reproduced thus far, siring 15 offspring, creating a small sample to make strong inferences. Our preliminary findings based on this small sample suggest that males who affiliate with and direct aggression to specific females gain a reproductive advantage with those females, but additional data are clearly needed. As these data accumulate, evaluating the effects of affiliation and aggression on male reproduction will be complicated because additional factors that we have not considered will require examination. For instance, we are likely to have underestimated the importance of sexual coercion, as it may act to ensure mating exclusivity as well as increasing a male's ability to mate with a specific female [12]. In this context, aggression is often used to initiate consortships in chimpanzees, where males lead females away from other community members and mate with them exclusively for several days (e.g. [21]). The challenge of maintaining exclusivity is not uniform. It may be relatively easy for high-ranking males because fewer males will challenge them to mate, or easier to accomplish with nulliparas, who are not preferred mating partners [47]. Second, we conducted this study over a relatively short period spanning two years, which covered only a single reproductive cycle for most females. Additional research is required to determine whether affiliative bonds between males and females endure and whether the patterns of aggressive and affiliative behaviour between bonded pairs persist and impact male reproduction over the long term (e.g. [30,37]). Determining whether such long-term relationships exist and how they impact male mating and reproductive success will improve our understanding of male chimpanzee development and the functional consequences of their behaviour.

Our findings also provide insights into the evolution of human pair bonds. Although the mechanisms that ensure paternity certainty in our species are diverse, including intimate partner violence [73–75] and larger cultural structures (e.g. religion: [76,77]), our finding that affiliative bonds between males and females appear entwined with sexual coercion in one of our two closest living relatives suggests that this aspect of intersexual relationships may be embedded deeply in our past.

Previous research suggests that choice causes an illusion of control—that it makes people feel more likely to achieve preferable outcomes, even when they are selecting among options that are functionally identical

Does Choice Cause an Illusion of Control? Joowon Klusowski, Deborah A. Small, Joseph P. Simmons. Psychological Science, January 5, 2021. https://doi.org/10.1177/0956797620958009

Abstract: Previous research suggests that choice causes an illusion of control—that it makes people feel more likely to achieve preferable outcomes, even when they are selecting among options that are functionally identical (e.g., lottery tickets with an identical chance of winning). This research has been widely accepted as evidence that choice can have significant welfare effects, even when it confers no actual control. In this article, we report the results of 17 experiments that examined whether choice truly causes an illusion of control (N = 10,825 online and laboratory participants). We found that choice rarely makes people feel more likely to achieve preferable outcomes—unless it makes the preferable outcomes actually more likely—and when it does, it is not because choice causes an illusion but because choice reflects some participants’ preexisting (illusory) beliefs that the functionally identical options are not identical. Overall, choice does not seem to cause an illusion of control.

Keywords: choice, illusion of control, open data, open materials, preregistered

General Discussion

Across 17 studies, we found no evidence that choice causes an illusion of control. Choice rarely made people feel more likely to achieve preferable outcomes when all options were functionally identical, whether we used different outcome measures (Studies 1–3), made the process visual (Study 4), varied the levels of uncertainty (Studies 5–7), or increased the subjectivity of the outcome evaluations (Studies 8 and 9). Choice had such effects only when it conferred actual control (Studies 10 and 11). In the rare cases in which choosers felt more likely to achieve preferable outcomes (Studies 12–15), choice seemed to reflect people’s preexisting beliefs rather than cause an illusion (Studies 16 and 17).

Our findings that a purported effect of choice results from an alternative account shares similarities with other reinvestigations of classic findings. Specifically, Chen and Risen (2010) showed that what looks like a choice-driven attitude change via cognitive-dissonance reduction in fact occurs because the choice is used to select people with different attitudes in the first place. Similarly, Tong, Feiler, and Ivantsova (2018) showed that what appears as choice-driven overoptimism via motivated reasoning emerges because choice reveals options that people already overestimate. Likewise, we revisited the highly influential and widely accepted phenomenon that choice causes an illusion of control. We found that such patterns rarely occur in cleanly designed experiments, and when they do, they are due to the choice reflecting people’s preexisting beliefs rather than causing an illusion. Together, this line of work suggests that some purported effects of choice may be due to the choice acting as a selection mechanism—among either different participants or different options—rather than as a cause of such effects.

Limitations

Despite our attempts to provide a comprehensive investigation, multiple questions remain.

First, our research does not address what might moderate the difference between the nonsignificant (Studies 1–11) and the significant (Studies 12–15) effects of choice. One possibility is that the probability estimates used in the latter studies are more sensitive than the other measures. In fact, our result from Study 15, which directly compared the probability estimates and multiple-choice measures, seems consistent with this conjecture. However, the nonsignificant coefficients in Studies 1 to 9 do not have a consistently positive sign, which is not what one would expect if the measures were merely less sensitive. Moreover, Studies 10 and 11 directly show that these measures were sensitive enough to respond when the choice made the preferable outcomes actually more likely. Another possibility is that evaluating multiple options in Studies 12 to 15 makes people more likely to develop normatively incorrect beliefs and hence more likely to show patterns that appear consistent with a choice-driven illusion of control. When one evaluates multiple options, there is a greater number of ways to express normatively incorrect beliefs (vs. only one way to express the normatively correct belief) than when one evaluates a single option, which might facilitate such beliefs. However, our results from Studies 12 to 14 indicate that the number of evaluated options does not always moderate the effect of choice. Although our research did not address these puzzling discrepancies, subsequent research could examine what may explain them.

Second, we do not know what led a subset of participants to demonstrate preexisting illusions, even when we explicitly informed them that all options have identical prospects (Studies 16 and 17). It is possible that certain individuals were prone to forming these beliefs because of past experiences, superstitious thinking, or distrust, but our research did not address what causes these beliefs or whether they are generalizable to people outside of our samples (Harris & Osman, 2012; Risen, 2016, Sharpe, Adair, & Roese, 1992). Future research could examine what individual or situational factors can lead people to develop such beliefs in the face of instructions that contradict them.

Third, our research focused on choice and thus did not address other factors that might truly cause an illusion of control. Previous research suggests additional factors that might cause an illusion of control (e.g., competition, familiarity, active involvement; Langer, 1975; Martinez et al., 2009). Although our research suggests that choice is unlikely to cause an illusion of control itself, it is possible that these other factors could.

In conclusion, past research suggests that choice can be powerful even without conferring actual control because it creates an illusion of control. Our research suggests a more sober perspective on the value of choice: Choice simply enables people to get what they want.

Empathy for joy recruits a broader prefrontal network than empathy for sadness; prefrontal cortex activation during both types of empathy was positively predicted by working memory capacity

Taiwo, Z., Bezdek, M., Mirabito, G., & Light, S. N. (2021). Empathy for joy recruits a broader prefrontal network than empathy for sadness and is predicted by executive functioning. Neuropsychology, 35(1), 90–102. https://doi.org/10.1037/neu0000666

Rolf Degen's take: https://twitter.com/DegenRolf/status/1346485527550320641

Abstract: Empathy encompasses the ability to contemplate and vicariously share in the emotional life of others, and is critical for social interaction, and may enhance subjective happiness.

Objective: While a few theoretical models propose that executive function may play a role in empathy, it is unknown how variation in executive function, and underlying variation in key large-scale brain network nodes, such as the dorsolateral prefrontal cortex node within the executive control network—or the medial prefrontal cortex (PFC) node within the mentalizing/theory of mind network—may account for individual differences in empathy capacity.

Method: The relationship between individual differences in executive capacity—parsed into working memory, inhibition, and cognitive flexibility subdomains—and magnitude of activity in a priori identified PFC subregions during a functional MRI-based ecologically valid empathy induction paradigm, was investigated. Empathic happiness (i.e., vicarious joy) and empathic concern (i.e., vicarious sadness) in response to the life circumstances of actual people were measured at separate time points as brain functional MRI was obtained. Participants also completed executive-heavy clinical neuropsychological tasks outside of the scanner.

Results: Frontopolar PFC was activated across both types of empathy. However, empathic happiness related to engagement of a much broader network of prefrontal cortex subregions relative to empathic concern: spawning frontopolar, dorsolateral, and medial aspects. PFC activation during both types of empathy was positively predicted by working memory capacity.

Conclusion: Activation in core aspects of the working memory-executive control network, and core happiness-related aspects of the mentalizing brain network (i.e., medial PFC and precuneus) predicted greater empathy capacity.

Documenting individual differences in the propensity to hold attitudes with certainty

DeMarree, K. G., Petty, R. E., Briñol, P., & Xia, J. (2020). Documenting individual differences in the propensity to hold attitudes with certainty. Journal of Personality and Social Psychology, 119(6), 1239–1265. https://doi.org/10.1037/pspa0000241

Abstract: The certainty with which people hold their attitudes is an important consideration because attitudes held with certainty better predict judgment and behavior than attitudes held with doubt. However, little is known about whether people’s assessments of their certainty reflect a disposition to hold attitudes with confidence. Adapting methods used to document individual differences in people’s attitudes, the present research demonstrates that the certainty with which people hold any given attitude is in part a reflection of a relatively stable disposition. Across 5 studies and 6 samples (total N = 106,050), we demonstrate dispositional variability in attitude certainty and show that it is related to but distinct from confidence in other judgmental domains. We also demonstrate that dispositional attitude certainty may be useful in predicting certainty in newly formed evaluations (Study 3) and an important consequence of certainty—attitude-behavior correspondence (as indicated by reports of behavioral intentions and recent behavior; Study 4 and Student Sample in Study 5). Furthermore, we demonstrate that dispositional attitude certainty is relatively stable over time (Study 5). Results are discussed with respect to potential mechanisms and boundary conditions relating to dispositional attitude certainty, the implications of these individual differences for attitudes and persuasion, as well as the potential origins of dispositional attitude certainty.