A new study published in Evolutionary Psychological Science challenges the theory that early fertility primarily benefits women from disadvantaged socioeconomic backgrounds. The research found that early fertility was associated with greater lifetime reproductive success regardless of childhood socioeconomic status. This finding diverges from the hypothesis that accelerated reproductive timing is an adaptive response to childhood adversity.
The study, based on the PhD dissertation of Atsushi Kometani, now an assistant professor at Kobe University, aimed to test a key concept in evolutionary psychology: the psychosocial acceleration theory. This theory suggests that early-life adversity, such as growing up in poverty or experiencing family instability, signals a harsh and unpredictable environment. In response, individuals may adopt a “fast” life strategy, prioritizing reproduction over long-term planning to maximize their chances of passing on their genes.
“Psychosocial acceleration theory posits that human females have an evolved reaction norm whereby women accelerate their reproductive timing in response to childhood harshness,” explained Yohsuke Ohtsubo, a professor in the Department of Social Psychology at the University of Tokyo and the corresponding author of the study.
“If this theory is valid, we expect that women who experienced adversity in their childhood would have more children by having their first babies earlier, but this would not hold for women who did not experience adversity. Although many studies show the hypothesized correlation between childhood adversity (e.g., father absence) and early reproductive timing (e.g., earlier menarche), few studies have tested whether early reproductive timing selectively benefits women from harsh environments.”
The study examined two independent groups of women: 480 participants from Japan and 780 from the United States, all aged 45 to 50. This age range was chosen because participants were likely to have completed their reproductive years, allowing researchers to estimate lifetime reproductive success, measured as the total number of biological children.
Participants completed surveys measuring their socioeconomic status during childhood, adolescence, and early adulthood. Socioeconomic status was assessed using self-reported indicators, such as financial stability and access to resources during those life stages. The researchers also gathered data on participants’ age at first birth, a key indicator of reproductive timing.
The analysis tested several hypotheses, including whether early fertility conferred reproductive advantages depending on socioeconomic background. To explore this, researchers compared lifetime reproductive success among women who had children early versus later, accounting for variations in childhood socioeconomic status.
The results contradicted the hypothesis that early fertility offers distinct advantages for women from low socioeconomic backgrounds. Women who had their first child at a younger age tended to have more children overall, regardless of whether they came from low or high socioeconomic status during childhood, in both Japan and the United States.
Women from higher socioeconomic backgrounds reported better general health later in life, aligning with previous research linking early-life adversity to poorer health outcomes. However, health measures did not directly mediate the relationship between early reproduction and lifetime reproductive success. This indicates that while childhood socioeconomic conditions may influence long-term health, they do not appear to significantly alter the reproductive advantages associated with early fertility.
“Some evolutionary hypotheses are intuitively appealing,” Ohtsubo told PsyPost. “It sounds reasonable that women in harsh environments could maximize their chance to leave some offspring by having their first babies earlier. Intuitive appeal does not imply correctness. As for the psychosocial acceleration theory, despite its intuitive appeal, there is little direct evidence showing that it is, in fact, an evolved life history strategy of humans.”
The researchers also examined whether the consistency of socioeconomic environments between childhood and early adulthood influenced reproductive outcomes. The hypothesis was that women whose adult socioeconomic status aligned with their childhood environment might achieve better reproductive success due to greater predictability.
However, the study found no significant evidence for this relationship. The consistency of socioeconomic conditions between life stages did not appear to enhance the number of children a woman had. This suggests that early fertility’s reproductive benefits operate independently of stability or predictability in an individual’s socioeconomic circumstances over time.
“Although it is not directly related to the psychosocial acceleration theory itself, we were surprised to find that the association between age at first birth and number of children around age 45 years has much commonality between the United States and Japan, despite many differences in their cultural and societal systems (e.g., lower divorce rate in Japan),” Ohtsubo said.
As with all research, there are limitations to consider. The study primarily measured childhood adversity through socioeconomic status, which may not fully capture the range of adversities individuals face. Other forms of adversity, such as exposure to violence, family instability, or community-level risks, were not included. These factors could influence reproductive timing and success in ways not accounted for in this study.
Additionally, the study evaluated reproductive success through the total number of children women had by the age of 45–50. While this is a common metric in evolutionary research, it does not account for potential trade-offs between the quantity and quality of offspring. For example, delayed reproduction might allow parents to invest more resources in fewer children.
“Although our ultimate goal is to test whether the reaction norm hypothesized by the psychosocial acceleration theory, in fact, evolved, we admit that we cannot strictly test it in modern environments,” Ohtsubo noted. “An ideal study would be conducted, for example, in Africa 20,000 years ago. We admit that it is very difficult (maybe impossible) to determine whether the reaction norm evolved. Our findings, along with similar findings in pre-industrialized societies (and baboon societies), can only cast doubt on the plausibility of the psychosocial acceleration theory.”
“We are afraid that evolutionary hypotheses are misinterpreted—that early fertility (especially that of women in poverty) is good for them because it is an ‘adaptive’ strategy, although ‘adaptive’ in evolutionary hypotheses has nothing to do with the well-being or health of women. One of our long-time goals is to provide a balanced explanation for early fertility. For example, we believe that early fertility of women in developing countries (especially women suffering from poverty) is better explained by societal habits (e.g., child marriage). We would like to establish reasonable boundary conditions for evolutionary hypotheses.”
The study, “Effects of Accelerated Reproductive Timing in Response to Childhood Adversity on Lifetime Reproductive Success in Modern Environments,” was authored by Atsushi Kometani and Yohsuke Ohtsubo.
