Religious individuals had higher reproductive success (this association was especially pronounced in males); religiousness did not show associations with parental investment

Examining the link between religiousness and fitness in a behavioural ecological framework. Janko Međedović. Journal of Biosocial Science, November 26 2019. https://doi.org/10.1017/S0021932019000774

Abstract: In recent years there have been attempts to explain religiousness from an evolutionary viewpoint. However, empirical data on this topic are still lacking. In the present study, the behavioural ecological theoretical framework was used to explore the relations between religiousness, harsh environment, fitness (reproductive success and parental investment) and fitness-related outcomes (age at first birth, desired number of children and the romantic relationship duration). The data were collected from 461 individuals from a community sample who were near the end of their reproductive phase (54% females, Mage = 51.75; SD = 6.56). Positive links between religiousness, harsh environment, fitness and fitness-related outcomes were expected, with the exception of age at first birth, for which a negative association was hypothesized. Hence, the main assumption of the study was that religiousness has some attributes of fast life-history phenotypes – that it emerges from a harsh environment and enables earlier reproduction. The study findings partially confirmed these hypotheses. Religiousness was positively related to environmental harshness but only on a zero-order level. Religious individuals had higher reproductive success (this association was especially pronounced in males) but religiousness did not show associations with parental investment. Religiousness was positively associated with desired number of children and negatively associated with age at first birth, although the latter association was only marginally significant in the multivariate analyses. Finally, path analysis showed that desired number of children and age at first birth completely mediated the relation between religiousness and reproductive success. The data confirmed the biologically adaptive function of religiousness in contemporary populations and found the mediating processes that facilitate fitness in religious individuals. Furthermore, the findings initiate a more complex view of religiousness in a life-history context which could be fruitful for future research: a proposal labelled as ‘ontogeny-dependent life-history theory of religiousness’.


Discussion

The behavioural ecological framework enables the analysis of the evolution of any behavioural trait ifthe trait in question is genetically transmitted across generations. This can even be applied to com-plex, socially and culturally influenced traits such as religiousness. However, the trait can be targetedby natural selection only if it is related to evolutionary fitness. Furthermore, one of the fundamentalassumptions of behavioural ecology is that individuals adapt to their local environments. The presentresearch sought to explore the relations between religiousness and fitness, the potential mediators ofthis relation and the environmental conditions that could be involved in it. The study hypothesis wasthat religiousness is biologically adaptive (i.e. it is positively associated with fitness and other fitness-related outcomes, all except age of first reproduction where negative association was assumed) andthat it emerges from harsh environmental conditions. These hypotheses were only partially confirmed. However, the study data provide a broader and more comprehensive view of religiousnessin a behavioural ecological context, confirming its adaptiveness in a biological sense. Furthermore, it reveals some of the mechanisms that religious individuals use to achieve higher reproductive success.Finally, the results are implicative for the future life-history theory of religiousness.

The associations between fitness and fitness-related measures
From the viewpoint of behavioural ecology, it is very important to analyse the relations betweenmeasures connected with fitness. First of all, reproductive success and parental investment werefound to be uncorrelated in the present research. This is not unusual–in fact, a negative correla-tion could be expected since number of children should be negatively related to parental invest-ment in each of them; this is a major evolutionary trade-off called the‘quantity–quality trade-off’(Lawson & Mace,2009). The absence of a negative correlation probably stems from the fact thatthe research was conducted in a low-fertility population, while the magnitude of this trade-off ishigher in populations with elevated mean reproductive success (Rosset al.,2016).
Age at first birth was found to be negatively related to both fitness indicators. This findingconfirms earlier findings of a negative directional selection on the timing of first reproduction:individuals who have their first child earlier in their lifetime have higher overall fitness (Tropfet al.,2015; Sanjaket al.,2018). The desired number of children was positively related to bothfitness measures as well. At first glance, this may sound like a trivial finding, but actually it is veryimportant since it shows unique features of contemporary human evolution: fertility in humans isbased on, but far from completely determined by, intentional motivation and planning (Johnson-Hanks,2008). Furthermore, it is at least partially subject to conscious control via contraceptionand other birth control measures. Finally, the duration of the partner relationship is positively relatedto reproductive success and negatively related to age at first birth: the longer individuals are in aromantic relationship, the earlier they become parents and they have more children. It is importantto note that these links were unchanged when participants’age was controlled in the analysis. Thus,long-term mating is apparently evolutionarily adaptive. This is in line with the theories that assumethat long-term mating is a dominant mating pattern in humans since human offspring need elevatedcare and investment from both parents (Stewart-Williams & Thomas,2013). In sum, the obtaineddata regarding the relations between fitness-related outcomes are quite congruent with previous find-ings and life-histories of contemporary humans.

Behavioural ecology of religiousness
Religious individuals have been shown to desire a higher number of children at the beginning oftheir reproductive phase, and they have their offspring earlier in their lifetime (although this linkwas rather weak in the present research) and have higher total fertility in general (this association was pronounced particularly in males, but it did not reach statistical significance in a subsample of females). However, they did not show elevated parental investment. A positive relation betweenr eligiousness and parental investment was assumed since religiousness is related to a closenesstowards family members and family values in general (Jensen & Jensen,1993). The absence ofthis link may suggest that religious individuals are oriented towards offspring quantity but notnecessarily offspring quality as a way of optimizing fitness.A positive link between religiousness and reproductive success has been empirically obtained inprevious research (Sanderson,2008; Blume,2009; Fieder & Huber,2016). The present study alsofound a positive link between religiousness and the desired number of children. These data are inline with a previous finding that shows positive attitudes of religious individuals towards child-bearing (Hayford & Morgan,2008). Furthermore, major religions often advocate a higher family size (Sanderson,2008). Previous research has also obtained evidence that religious individuals tend to have their first child earlier in their lifetime (Pearce & Davis,2016). This was confirmedin the present study, although the link was relatively weak (i.e. only marginally significant in mul-tivariate analyses). Finally, religiousness may enable high fitness in a somewhat indirect way: byfacilitating longer romantic relationships via commitment to marriage, marital satisfaction and lower risk of divorce (Mahoneyet al., 2002). However, this link was not detected in the present data and this was the only fitness-related outcome that was not associated with religiousness. It isimportant to note that desired number of children and age of first birth completely mediated the link between religiousness and reproductive success. This was not expected due to a fact thatthere could be other mediators of this link; however, this result only highlights the role these twovariables have in elevating the fitness of religious individuals.In sum, the data obtained in the present research are in line with previous results suggesting that religiousness is probably under positive directional selection on fertility. Thus, selection actspositively on the genetic basis of religious attitudes. Note that this does not necessarily mean that higher phenotypic levels of religiousness in the upcoming generations should be necessarilyexpected. Many complex cultural and environmental factors act on the phenotypic developmentof religious attitudes and some of them may be opposed to selection. This is why the frequency of religious commitment has in fact been found to fall in Western populations (Zuckerman,2015).The complexity of the biological and environmental factors that shape religiousness prevents theprediction of its phenotypic levels in future populations.Towards a future life-history theory of religiousnessPrevious findings of negative associations between religiousness, sexual permissiveness andrestricted sexuality together with positive associations with serial monogamy suggest that religiousness is part of a slow life-history trajectory (Gladdenet  al.,2009; Baumard &Chevallier,2015; Schmitt & Fuller,2015). However, this view may be oversimplified. If religiousness emerges from a harsh environment and enables earlier reproduction this would mean that ithas the characteristics of the fast life-history trait as well. These associations were obtained in thepresent research although they were fragile. The positive link between harsh environment andreligiousness was heavily dependent on the participants’ sex and age.  The negative link between religiousness and age at first birth was low in magnitude and marginally significant. However, these associations have been found in previous studies as well, and with more convincing effect sizes (Delamontagne, 2010; Pearce,2010; Soltet al.,2011; Pearce & Davis, 2016). It should be noted that elevated offspring quantity, which is clearly associated with religiousness, is the most important indicator of a fast life-history pathway in the first place. All these data suggest that religiousness indeed has some attributes of a fast life-history trajectory.

The present study was cross-sectional by design, which prevented making conclusions aboutthe causal relations between the measures. However, perhapsa  hypothesi sof religiousness’s involvement in life-history trajectories can be made. The existing data suggest that the life-history characteristics of religiousness are contingent on the stages of ontogeny. In earlier stages of development religiousness delays mating activity (expressed, for example, in negative associations between religiousness and the onset of sexual behaviour: Jones et al.,2005), which means that it has slow life-history attributes. However, in the reproductive stage itself, it is associated with earlier marriage and reproduction, thus acting as a fast life-history phenotype. When family is constituted, religiousness again turns to the slow life-history trait by decreasing sexual permissiveness and pro-moting monogamy. Hence, the life-history characteristics of religiousness are different during theontogeny. This proposition may be labelled as an‘ontogeny-dependent life-history theory of religiousness’. This hypothesis may be tested in future studies using a longitudinal approach.