Tomas Rees points to an interesting paper by Marcus Jokela, who examined how the fertility rates of Americans born between 1920 and 1960 were affected by their personality.

Using the big five personality traits - openness, conscientiousness, extraversion, agreeableness and neuroticism - Jokela found that higher levels of conscientiousness in women and higher levels of openness in both sexes became more strongly related to low fertility as time went on. Effectively, cultural conservatives are now more likely to have higher fertility.

Using this result, Rees argued that religiosity is not evolutionarily advantageous due to higher fertility, even though current fertility trends may make it seem so. Rather, the link is between conservative values and fertility, and this link is relatively recent.

I appreciate this argument, largely because it highlights the inconsistency and looseness in the way many studies define phenotypes. Is religiosity just a manifestation of someone’s big five personality traits? We consistently see new papers about the heritability and evolutionary advantage of various political views, happiness, religious persuasions, income and so on. But ultimately, many of these analyses are versions of the same theme, and the analysis could be hardened by deciding on some firm phenotypic traits on which to conduct the analysis. For example, most religious views can probably be captured through the big five personality traits and IQ. I have previously made a similar point about the heritability of political views. If the analyses were linked to a consistent set of phenotypic traits, we would be better positioned to compare results, look at them through time and to explore the underlying causal mechanisms.

Jokela’s paper also highlights that a trait may become advantageous or not with changes in the environment. Heritable variation in reproductive outcomes is often evidence that the evolutionary advantage is relatively recent. After all, evolution eliminates variation. As a result, particularly in modern contexts, we need to consider what environmental shocks have played out. We can make a coherent story about Jokela’s findings using that framework, with changes in the social environment between 1920 and 1960 allowing those who were more open to experience to choose the alternative low fertility option.