What the study of spinal cord injured patients can tell us about the significance of the body in cognition

What the study of spinal cord injured patients can tell us about the significance of the body in cognition. V. Moro, M. Scandola & S. M. Aglioti. Psychonomic Bulletin & Review, Jun 13 2022. https://rd.springer.com/article/10.3758/s13423-022-02129-6

Abstract: Although in the last three decades philosophers, psychologists and neuroscientists have produced numerous studies on human cognition, the debate concerning its nature is still heated and current views on the subject are somewhat antithetical. On the one hand, there are those who adhere to a view implying ‘disembodiment’ which suggests that cognition is based entirely on symbolic processes. On the other hand, a family of theories referred to as the Embodied Cognition Theories (ECT) postulate that creating and maintaining cognition is linked with varying degrees of inherence to somatosensory and motor representations. Spinal cord injury induces a massive body-brain disconnection with the loss of sensory and motor bodily functions below the lesion level but without directly affecting the brain. Thus, SCI may represent an optimal model for testing the role of the body in cognition. In this review, we describe post-lesional cognitive modifications in relation to body, space and action representations and various instances of ECT. We discuss the interaction between body-grounded and symbolic processes in adulthood with relevant modifications after body-brain disconnection.

(Dis)Embodied approaches to cognition

The traditional occidental concept of the human mind seems to be essentially based on mind-body dualism deriving from the Cartesian distinction between the mind (res cogitans) and the body (res extensa). The mind-body dichotomy has been taken to imply not only that basic perceptual and motor functions are separated from higher order ones (Block, 1995), but also that the latter are exclusively based on the manipulation of abstract, amodal symbols and are largely independent from the former (Newell & Simon, 1972). In the last few decades, this radical view has been challenged by ever increasing psychological and neuroscientific evidence that human cognition is profoundly influenced by basic sensorimotor processes and that even complex concepts such as the abstract aspects of language are largely grounded on body representations and their relations with the world. This is the central tenet of a group of theories that are included under the umbrella definition of ‘Embodied Cognition Theories’ (ECTs). According to these theories, all human experience is grounded in the body, not only perceptual and emotional processes and social interactions, but also the acquisition and creative use of language (e.g., the use of metaphors), judgment capacities and the creation of cultural artefacts (Gallagher, 2005). Since their original formulation (Glenberg, 1997), ECTs have attracted the interest of many disciplines, such as psychology, psychotherapy (Khoury et al., 2017; Tschacher et al., 2017), education (Pouw et al., 2014), philosophy, anthropology, robotics (Hoffmann et al., 2010), artificial intelligence (Shapiro, 2011) and, last but not least, neuroscience (Freund et al., 2016; Kiefer & Pulvermüller, 2012; Mahon & Caramazza, 2008). However, ECTs do not refer to a unitary construct and each theory does in effect differ from another in the way it conceives the reciprocal relations between the body, the mind and the environment and the modalities by means of which bodily representations affect cognition. The various different theories range from a general idea of an instrumental role of the body in information processing (Körner et al., 2015) to a more radical view asserting that “all cognitive processes are based on sensory, motor and emotional processes, which are themselves grounded in body morphology and physiology” (Glenberg, 2015, p. 166).

Importantly, however, a sort of continuum is identifiable within these various theories (Fig. 1). At one extreme of this continuum, there is a hypothesis that presupposes the hierarchical organisation of cognition with a symbolic system that is separated from the sensorimotor system that can merely activate motor responses (Leshinskaya & Caramazza, 2016). At the other extreme is the idea that cognition emerges from a dynamic circle of interactions between the brain, the body, and the environment without the need for symbols (Brooks, 1991; van Gelder, 1998). What distinguishes these two perspectives regards the role that the body and its connection to objects plays in cognition (Shapiro, 2019). The body may be considered to ‘participate’ in building cognition since cognition may be altered depending on the shape, size and experiences of the body (Glenberg, 1997; Lakoff & Johnson, 1999; Varela et al., 1991). From a different perspective, the body can be considered to be ‘constitutive’ in the sense that cognition would not exist without it (e.g., the Perceptual Symbol theory; Barsalou, 2008; O’Regan & Noë, 2001). Objects are only taken into account in some of these theories in which it is suggested that they participate in building cognition (e.g., the Extended mind theory, Clark, 2006; the Dynamical systems theory, van Gelder, 1998). An example is the act of writing and thinking at the same time, a task that gives a specific result due to the interaction between the brain and the body and thence to a pen and paper, and from there back again to the brain (Clark, 2006). Accordingly, if one changes either the gesture or the object, the final product will also be different. One might ask whether in this case the mind extends to the body (e.g., the Peripheral mind theory, Aranyosi, 2013) and also to the objects (Clark, 2006) or, alternatively, the mind incorporates the body and the objects it is interacting with (Borghi, 2005). This is a question that remains unanswered.

Fig. 1

The various different models of embodied cognition theories are represented in a progression from one extreme with Disembodied Cognition to the other extreme positions inside the Embodied Cognition Theories. The co-existence of modal and amodal symbols in adulthood is suggested

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Recent studies on the link between embodiment and higher order functions in people with sensory deprivation highlight the importance of both sensory and conceptual representations (Ostarek & Bottini, 2021). For example, anterior temporal lobe activation in colour-knowledge tasks turned out to be very similar in congenital and early blind subjects (Wang et al., 2020). In contrast, activation in the ventral occipito-temporal colour perception regions was found only in sighted controls. This pattern of results points to the existence of two forms of object representation in the human brain: a sensory-derived and a cognitive-derived form of knowledge (Wang et al., 2020), with the former being experience-dependent and the latter experience-independent (Ostarek & Bottini, 2021). Crucially, the analyses of connectivity in Wang et al.’s study shows that the two systems relating to colour knowledge are integrated and part of a widespread network (Wang et al., 2020). Thus, a crucial question concerns not only whether but also how the two levels interact and if the sensory level is able to modulate and modify the conceptual level. If so, one can conclude that knowledge is embodied, although embodiment is not the only way the brain understands the world.

While no single clinical condition makes it possible to distinguish between the various different ECTs, alterations in the body may provide novel information on the different variables that play a role in these processes. Studies of amputees, for example, may highlight possible representational bodily changes that might, however, be due to multiple aspects, such as the visual appreciation of conspicuous changes in body shape as well as the somatosensory and motor disconnection between the body and the brain. In the following section, we focus on individuals suffering from spinal cord injury (SCI) in whom the general body shape is unchanged in spite of a massive somatosensory de-afferentation and motor de-efferentation. The specificity of this neurological model with respect to other clinical conditions will be analysed, then the changes in cognitive functions associated with SCIs are reviewed, starting from the representation of static and acting bodies, and continuing with an exploration of object and space representations. The potential contribution of these experimental data to the debate on embodied cognition will conclude the review.

Sweet memories have extraordinary potential to relieve the bitterness of life

The power of negative and positive episodic memories. Samantha E. Williams, Jaclyn H. Ford & Elizabeth A. Kensinger. Cognitive, Affective, & Behavioral Neuroscience, Jun 14 2022. https://rd.springer.com/article/10.3758/s13415-022-01013-z

Abstract: The power of episodic memories is that they bring a past moment into the present, providing opportunities for us to recall details of the experiences, reframe or update the memory, and use the retrieved information to guide our decisions. In these regards, negative and positive memories can be especially powerful: Life’s highs and lows are disproportionately represented in memory, and when they are retrieved, they often impact our current mood and thoughts and influence various forms of behavior. Research rooted in neuroscience and cognitive psychology has historically focused on memory for negative emotional content. Yet the study of autobiographical memories has highlighted the importance of positive emotional memories, and more recently, cognitive neuroscience methods have begun to clarify why positive memories may show powerful relations to mental wellbeing. Here, we review the models that have been proposed to explain why emotional memories are long-lasting (durable) and likely to be retrieved (accessible), describing how in overlapping—but distinctly separable—ways, positive and negative memories can be easier to retrieve, and more likely to influence behavior. We end by identifying potential implications of this literature for broader topics related to mental wellbeing, education, and workplace environments.

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Positive memories also hold a power all their own. Unlike the affect associated with negative memories, which tends to fade relatively quickly, positive memories are more likely to retain their affective intensity (Walker et al., 1997; see recent review by Skowronski et al., 2014). This may be part of the reason why positive autobiographical memories act as rewards in themselves (Speer et al., 2014) and can buffer effects of stress (Speer & Delgado, 2017). Memories for positive personal events become more integrally tied to our sense of self and can perpetuate self-esteem (Çili & Stopa, 2015) and become an important part of our life story (Berntsen et al., 2011).

Given these features of positive autobiographical memories, it may come as no surprise that they have high utility and can be strategically recalled to good purposes (Figure 3). Positive memories are powerful in their ability to repair our moods after a negative mood induction (Joormann et al., 2007; Joormann & Siemer, 2004), to connect us socially (Rasmussen & Berntsen, 2009; Wolf & Demiray, 2019), and to inspire us toward prosocial behavior (Gaesser & Schacter, 2014). By activating reward circuitry, they even may trigger mnemonic circuitry that increases the likelihood that we encode the good in the world around us. We will review the literature shedding light on the power of positive memories.

Studying two highly divergent phyla of worms that contain numerous parasites that cause human and livestock diseases, new research sheds light on how sexual reproduction and subsequent great diversity of sex chromosomes might have evolved

Yifeng Wang, Robin B. Gasser, Deborah Charlesworth, Qi Zhou. Evolution of sexual systems, sex chromosomes and sex-linked gene transcription in flatworms and roundworms. Nature Communications, 2022; 13 (1). Jun 10 2022. DOI: 10.1038/s41467-022-30578-z

Abstract: Many species with separate male and female individuals (termed ‘gonochorism’ in animals) have sex-linked genome regions. Here, we investigate evolutionary changes when genome regions become completely sex-linked, by analyses of multiple species of flatworms (Platyhelminthes; among which schistosomes recently evolved gonochorism from ancestral hermaphroditism), and roundworms (Nematoda) which have undergone independent translocations of different autosomes. Although neither the evolution of gonochorism nor translocations fusing ancestrally autosomal regions to sex chromosomes causes inevitable loss of recombination, we document that formerly recombining regions show genomic signatures of recombination suppression in both taxa, and become strongly genetically degenerated, with a loss of most genes. Comparisons with hermaphroditic flatworm transcriptomes show masculinisation and some defeminisation in schistosome gonad gene expression. We also find evidence that evolution of sex-linkage in nematodes is accompanied by transcriptional changes and dosage compensation. Our analyses also identify sex-linked genes that could assist future research aimed at controlling some of these important parasites.

Popular version: Parasitic worms reveal new insights into the evolution of sex and sex chromosomes Two worm phyla give clues on how sex chromosomes might have evolved. Jun 15 2022. https://www.sciencedaily.com/releases/2022/06/220615102849.htm

Discussion

The evolution of sex chromosomes in some taxa involves the primary transition from a hermaphroditic system or from environmental sex determination to a dioecious/gonochoristic species with genetic sex determination. Such transitions usually are accompanied by potential suppression of recombination in and around the sex-determining gene(s), and the recombination suppression sometimes even extends to other sex chromosome regions. In other species, turnover events may create new sex-determining regions, which may also evolve into non-recombining regions. In either case, autosomal regions that fused or translocated to sex chromosomes may sometimes also become completely sex-linked, either in species that lack recombination in the heterogametic sex26, or potentially in species with recombination in both sexes through subsequent recombination loss.

Schistosomes have evolved gonochorism from hermaphroditism and exhibit strong morphological sexual dimorphism (see above). As explained earlier, the first step in such a primary transition must either involve a mutation creating females (producing a gynodioecious population), or one creating males (producing an androdioecious population). A mutation in the highly conserved oogenesis-related or feminising gene mag-1 might have produced males in the ancestor of schistosomes. However, it seems unlikely that such a mutation could have greatly increased male fitness, compared with that of the ancestral hermaphrodite, as required for the establishment of androdioecy. Females could have arisen due to a dominant mutation in the reported W-linked candidate sex-determining gene U2AF2 (Fig. 5k)85,86. The involvement of these genes in the evolution of schistosome sex-determination needs to be tested in the future.

Our finding that the evolution of the present schistosome sex-linked regions was followed by transcriptional changes of many genes in gonads (Fig. 5d–g) is consistent with the hypothesis of sexual antagonism in the hermaphroditic ancestor, favouring re-allocation of resources after separate sexes evolved75. Assuming that higher transcription levels reflect advantageous changes, the results in schistosomes suggest that conflicts were resolved and a new optimum reached more frequently in males (masculinisation and overwhelming defeminisation) than in females (feminisation) in the gonads. In the transition to dioecy in Silene latifolia, with male, instead of female heterogamety, and an XY sex chromosome system, transcriptional changes occurred most frequently in females92. The results suggest transcriptional changes after the X or Z chromosome became hemizygous in one sex, resulting in masculinisation of the schistosome Z (as shown in Supplementary Fig. 16) and feminisation of the X chromosome in S. latifolia.

Following the origin of gonochorism, both schistosome and nematode ancestral sex chromosomes have undergone translocations of autosomes, like those in many other taxa26,56 (Figs. 3 and 5a). The translocated autosomes, or large parts of them, have become completely non-recombining in both phyla, and in nematodes they have become strongly degenerated like the ancestral sex chromosomes. How loss of recombination happened is an interesting question. Recombination between the autosomes involved in a fusion or translocation with sex chromosomes often maintain autosomes’ former recombination patterns. However, the study of fusions between the C. elegans X chromosome and chromosome IV62 suggested that crossovers may be re-positioned away from the fusion junction, creating a new chromosome with two arm regions (whereas the two participating chromosomes each contained two arm regions). In the fused chromosome, a potentially large former arm region close to the fusion point may thus have greatly reduced recombination, and if the fusion involves the X chromosome, this will occur specifically in males. Such events can therefore create new sex-linked regions without involving selection for suppressed recombination.

Translocations of autosomes to ancestral sex chromosomes may be common in nematodes because some, though not all, nematodes have holocentric chromosomes93. Such chromosomes may be more prone to fusions or fissions than monocentric ones, in which such rearrangements may lead to a loss or multiplication of centromeres94. However, a recent comparison of insects with different centromere types found no evidence supporting this hypothesis95.

Finally, we annotated many Y- or W-linked genes additional to those already known in the flatworm and roundworm species studied here (Figs. 4 and 5). We also found homologs of C. elegans sex-determination pathway genes that may have undergone duplications in different nematode species. These genes could be involved in the divergence of the sex-determination pathways, as has already been documented between C. elegans vs. C. briggsae96. Functional verification in other nematode species is needed in the future. The present study could not identify further changes that may have occurred after lineage-specific duplications of these genes, and possible changes also need to be studied further. For example, gld-1 was independently recruited into the sex-determination pathways of C. elegans and C. briggsae; in C. elegans it acts to promote spermatogenesis, but it promotes oogenesis in C. briggsae97. Its co-factor, fog-2, evolved by a duplication and acquisition of a new GLD-1-binding domain in C. elegans88,98. The newly annotated, candidate sex-determining genes could be a useful resource for future studies of parasite control through interfering with their sexual life cycles.

We cannot read the generosity of others from their facial features, but we are convinced that the good looking are more generous

You Cannot Judge a Book by Its Cover: Evidence from a Laboratory Experiment on Recognizing Generosity from Facial Information. Ninghua Du, Fei Song, C. Bram Cadsby. Journal of Behavioral and Experimental Economics, June 14 2022, 101909. https://doi.org/10.1016/j.socec.2022.101909

• People cannot glean information about other-regarding preferences from facial information

• People make systematic errors when they try to identify generosity from facial information

• Those who are rated as more attractive are perceived to be more generous

• There is no actual relationship between physical attractiveness and generosity

Abstract: People form first impressions of others and may make judgments about their social traits and character on the basis of facial perceptions. We implement a controlled laboratory experiment to investigate whether people can glean information about another person's other-regarding preferences from uncropped photographs of their face. To do so, we conduct a dictator game with an allocator and a recipient, and then present pairs of allocator photos to observers. Each pair portrays one relatively generous allocator and another who has demonstrated less generosity. The experimental results show that the observers cannot accurately recognize more generous allocators, but instead make systematic errors. In particular, the observers believe that allocators who are rated more attractive by others are more generous, despite there being no actual relationship between physical attractiveness and generosity.

Keywords: Face-based JudgmentExperimentDictator GameOther-regarding PreferencesGenerosityAppearance

Feeling younger than one's chronological age was associated with better mental and physical health, presence of meaning in life, successful aging, optimism, personal mastery, resilience, curiosity, hope, and social support

Subjective age and its relationships with physical, mental, and cognitive functioning: A cross-sectional study of 1,004 community-dwelling adults across the lifespan. Awais Aftab et al. Journal of Psychiatric Research, June 14 2022. https://doi.org/10.1016/j.jpsychires.2022.06.023

Abstract: Perceived younger age is associated with positive health outcomes in existing literature. Few studies have examined these associations using a wide range of variables in large sample of adults of all ages. The objective of present study was to characterize the discrepancy between chronological age (CA) and subjective age (SA) in a large sample of community-dwelling adults across the lifespan, investigate associations with mental, physical, and cognitive health, and examine how it is related to a broad array of psychosocial variables relevant to well-being. Cross-sectional data from 1,004 individuals aged 21–100+ years from the Successful AGing Evaluation (SAGE) study were used for this analysis. Data included self-report measures of physical health (SF-36 – Physical Component), mental health composite score (created using CES-D Happiness scale, Satisfaction with Life Scale, SF-36 Mental Component, Brief Symptom Inventory Anxiety Scale, Patient Health Questionnaire-9, and Perceived Stress Scale), Telephone Interview for Cognitive Status - modified (TICS-m), and validated measures of various positive psychological variables such as meaning in life and optimism. On average, SA was 11.5 years younger than CA (SD 11.3). The discrepancy increased with CA. A younger SA compared to CA was associated with better mental and physical health in all age groups and was positively associated with measures of presence of meaning in life, successful aging, optimism, personal mastery, resilience, curiosity, hope, and social support. The association between age discrepancy and cognitive functioning was not statistically significant. These findings indicate that SA is potentially valuable for the purposes of clinical assessment and intervention, and this possibility should be investigated in future research.

Keywords: Positive psychiatryMeaning in lifeSuccessful agingOptimismPersonal masteryResilience

High social status, good looks, and being well-off and influential did not belong to the traits found desirable in a friend, and women held higher expectations for friends than men

Friendship Preferences: Examining Desirable and Undesirable Traits in a Friend. Menelaos Apostolou & Panagiota Vetsa. Evolutionary Psychological Science, Jun 14 2022. https://link.springer.com/article/10.1007/s40806-022-00329-w

Abstract: Friendship constitutes a human universal, with people across different times and places forming friendly relationships. Yet, people are selective in whom they befriend. The current research aimed to identify friendship preferences, that is, the traits that people find desirable or undesirable in a friend. More specifically, Study 1 employed open-ended questionnaires and identified 50 traits that participants preferred their friends to have, and 43 traits that they preferred their friends not to have. Study 2 employed a sample of 706 Greek-speaking participants and classified desirable traits into 10 broader factors; the most important one was being honest, followed by being ethical, pleasant, and available. Study 3 employed a sample of 865 Greek-speaking participants and classified undesirable traits into three broader factors. The most undesirable one was being dishonest, followed by being competitive and being impatient. In both studies, women tended to give higher scores than men. In addition, significant age effects were found for most factors in both studies.

The biological basis of intelligence

The biological basis of intelligence: Benchmark findings. Kirsten Hilger et al. Intelligence, Volume 93, July–August 2022, 101665. https://doi.org/10.1016/j.intell.2022.101665

Highlights

• Focused overview of research on the biological basis of intelligence.

• Benchmark findings from EEG, neuroimaging, and genetic research.

• Critical open questions and future directions.

Abstract: The scientific study of the biological basis of intelligence has been contributing to our understanding of individual differences in cognitive abilities for decades. In particular, the ongoing development of electrophysiological, neuroimaging, and genetic methods has created new opportunities to gain insights into pressing questions, allowing the field to come closer towards a comprehensive theory that explains how genotypes exert their influence on human intelligence through intermediate biological and cognitive endophenotypes. The aim of this article is to provide a focused overview of empirical benchmark findings on biological correlates of intelligence. Specifically, we summarize benchmark findings from electrophysiological, neuroimaging, and genetic research. Moreover, we discuss four open questions: (1) The robustness of research findings; (2) the relation between neural parameters and cognitive processes; (3) promising methodological developments; and (4) theory development. The aim of this paper is to assemble the most important and robust findings on the biological basis of intelligence to stimulate future research and to contribute to theory development.

Keywords: NeuroscienceElectroencephalographyMagnet resonance imaging (MRI)GeneticsIntelligenceCognitive abilities

Can we blame social media for polarization? Counter-evidence against filter bubble claims during the COVID-19 pandemic

Can we blame social media for polarization? Counter-evidence against filter bubble claims during the COVID-19 pandemic. S Mo Jones-Jang, Myojung Chung. New Media & Society, June 13, 2022. https://doi.org/10.1177/14614448221099591

Abstract: Although collective efforts are essential to fight COVID-19, public opinion in the United States is sharply divided by partisan attitudes and health beliefs. Addressing the concern that media use facilitates polarization, this study investigated whether social and traditional media use for COVID-19 information attenuates or reinforces existing disparities. This article focuses on two important areas where the public is highly polarized: partisan affect and vaccine attitudes. Contradicting the filter bubble claim, our survey (n = 1106) revealed that social media use made people less polarized in both partisan affect and vaccine hesitancy. In contrast, traditional media use made people more polarized in partisan affect. These findings corroborate the growing evidence that social media provide diverse viewpoints and incidental learning.

Keywords: Affective partisan polarization, COVID-19, polarization, social media, vaccine hesitancy

Negative affect decreases the weight that participants place on the health benefits and on the taste of food

Negative Affect, Affect Regulation, and Food Choice: A Value-Based Decision-Making Analysis. Daniel O’Leary et al. Social Psychological and Personality Science, June 13, 2022. https://doi.org/10.1177/19485506221079947

Abstract: Maladaptive eating is one of the greatest threats to health and well-being in the 21st century. Psychological factors that drive maladaptive eating are of interest as they may offer low-cost intervention targets. One such factor is negative affect. If negative affect does lead to maladaptive eating, interventions that reduce negative affect should lead to improved eating and food choice. One relevant class of techniques is affect regulation strategies. In the present research, we use survey data and a value-based decision-making task to demonstrate that negative affect is associated with maladaptive eating and food choice. We find that negative affect decreases the weight that participants place on the health benefits of food. We also show that teaching participants to use reappraisal to downregulate negative affect leads to healthier food choices. These findings indicate that reappraisal applied to incidental negative affect may be an effective method for improving eating and food choice.

Keywords: food, eating, affect, affect regulation, self-control, decision-making, value

Many taxa show substantial differences in lifespan between the sexes: however, these differences are not always in the same direction

The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life. Zoe A. Xirocostas, Susan E. Everingham and Angela T. Moles. Biology Letters, March 4 2020. https://doi.org/10.1098/rsbl.2019.0867

Abstract: Many taxa show substantial differences in lifespan between the sexes. However, these differences are not always in the same direction. In mammals, females tend to live longer than males, while in birds, males tend to live longer than females. One possible explanation for these differences in lifespan is the unguarded X hypothesis, which suggests that the reduced or absent chromosome in the heterogametic sex (e.g. the Y chromosome in mammals and the W chromosome in birds) exposes recessive deleterious mutations on the other sex chromosome. While the unguarded X hypothesis is intuitively appealing, it had never been subject to a broad test. We compiled male and female longevity data for 229 species spanning 99 families, 38 orders and eight classes across the tree of life. Consistent with the unguarded X hypothesis, a meta-analysis showed that the homogametic sex, on average, lives 17.6% longer than the heterogametic sex. Surprisingly, we found substantial differences in lifespan dimorphism between female heterogametic species (in which the homogametic sex lives 7.1% longer) and male heterogametic species (in which the homogametic sex lives 20.9% longer). Our findings demonstrate the importance of considering chromosome morphology in addition to sexual selection and environment as potential drivers of sexual dimorphism, and advance our fundamental understanding of the mechanisms that shape an organism's lifespan.

4. Discussion

Our study provides evidence that, across multiple taxa, the heterogametic sex tends to have a considerably shorter lifespan than the homogametic sex. That is, an organism's chromosome morphology seems to have a substantial role in shaping this key life-history trait. The 17.6% difference between the lifespans of homogametic and heterogametic sexes revealed here is substantial enough to have major ecological and evolutionary implications. However, heterogametic sex chromosomes include everything from a complete absence of the second sex chromosome (X0 or Z0), to a highly reduced second sex chromosome (e.g. XY in humans), to X and Y or Z and W chromosomes of nearly equal length [5,32,33]. As not all heterogametic species have a degraded sex chromosome, our study likely represents a conservative test of the unguarded X hypothesis. A future direction will be to formally test the hypothesis that the difference in lifespan between sexes is proportional to the proportional difference in chromosome length between sexes. That is, to test the idea that species in which the second chromosome is absent or extremely reduced have a greater reduction in the lifespan of the heterogametic sex than do taxa in which the difference between sex chromosomes is relatively small. Ideally, this question should be addressed using a diverse range of taxa, both for generality, and to include species with as many different chromosome configurations, life histories and mating systems as possible. Another interesting direction for future research would be to begin to quantify the relative contributions of factors such as chromosome morphology, sexual selection, parental investment and exposure to predators.

Our second major finding was that when males are the heterogametic sex, they die 20.9% earlier than their female counterparts, but when females are the heterogametic sex, they die only 7.1% earlier than their male counterparts. Three possible explanations for this surprising trend include: (1) the degree of degradation of the Y chromosome, (2) telomere dynamics, and (3) side effects of sexual selection.

(1)	It is possible that the Y chromosome in male heterogametic species might tend to be more degraded than the W chromosome in female heterogametic species, potentially leading to a difference in heterogametic lifespan between XY and ZW systems. We know that many mammals (including humans) have highly reduced Y chromosomes [33–35]. There is also evidence that the relative length of the W and Z chromosomes can vary substantially even within clades (e.g. birds, snakes; [6,36–39]). However, a comparative analysis of the degradation of chromosomes across the tree of life has not yet been performed.
(2)	Telomeres are sections of non-coding DNA at the ends of chromosomes that protect coding DNA from deterioration during cell replication and other cellular processes [40,41]. Cell replication damages telomeres and studies suggest that the loss of telomere length over time causes the progression of ageing and shortening of lifespan [42]. However, oestrogen stimulates a promoter of the telomerase enzyme [43], which heals damaged telomeres by adding telomeric base pairs to its ends and indirectly activates other DNA repairing pathways [40]. Although we do not know whether oestrogen is important in all of our study species, it is possible that the effect of oestrogen on telomerase activation could help to explain the smaller decrease in lifespan when females are the heterogametic sex.
(3)	In many cases, males experience more intense sexual competition than females, as they are more reproductively efficient and so take more risks when pursuing a mating opportunity (e.g. males fighting for access to females or to establish their territory) [44,45]. Usually, females are not as efficient at reproducing, contribute more to their offspring than fathers, and so are predicted to engage in lower-risk behaviours [44–46]. Higher mortality in males owing to side effects of sexual selection, in combination with the effect of sex chromosomes on longevity, could also explain why there is a smaller lifespan difference between ZW females and ZZ males in comparison with XY males and XX females [11,15,44].

Understanding the mechanisms underpinning the substantial difference in lifespan dimorphism in male versus female heterogametic species is an important direction for future research, as this may improve our understanding of the factors that affect ageing. There is a multibillion-dollar industry in extending human lifespan [47], however, there is a crucial knowledge gap and we have much to learn about the basic biology underpinning longevity and the drivers of lifespan differences across sexes and species. Here, we have provided the first evidence that the heterogametic sex does, on average, die earlier than its homogametic counterpart across a range of taxa. We also found that lifespan dimorphism between the sexes is greater in male heterogametic species in comparison with female heterogametic species. These findings are a crucial step in uncovering the underlying mechanisms affecting longevity, which could point to pathways for extending life. We can only hope that more answers are found in our lifetime.