Recent research published in evolutionary psychology suggest that a neighborhood’s economic and demographic conditions can predict its residents’ reproductive patterns 15 years later. After analyzing millions of data points from the Canadian census, scientists found that areas with higher poverty rates and larger numbers of certain minority populations tend to have different rates of future family formation. These findings provide evidence that early environmental factors can shape long-term family planning trends, which may guide public health policy.
Vinicius Betzel Köhler, an adjunct faculty member in McMaster University’s Department of Psychology, Neuroscience, and Behavior, and Dr. Rutherford began the study to test an idea known as psychosocial acceleration theory. This concept derives from a broader biological framework called life history theory, which considers how organisms allocate their limited energy. Since humans cannot invest unlimited time and energy in everything, our bodies and minds unconsciously adapt to favor either physical growth or early reproduction depending on the environment.
“I’ve been interested in evolutionary psychology since I got my master’s degree,” Koehler said. “Life history theory and psychosocial acceleration theory (PAT) are two major concepts in Evo Psych that are currently undergoing serious consideration and critique.”
If these ideas are true, researchers should be able to observe the effects on a large scale, he explained. “If their hypothesis is correct, and in layman’s terms, humans have adapted to start their developmental clocks in response to the environments they encounter in the first few years of life, then we should see signs of that everywhere, including in the census,” Koehler said.
“It made sense to me that we needed to test the PAT hypothesis in the entire population,” Koehler added. “Another big motivation is that being able to use such theories and variables to predict trends in population reproduction would also be of incredible value to policy makers.”
Psychosocial acceleration theory suggests that growing up in a harsh and unpredictable environment triggers people to mature more quickly. Harshness generally refers to conditions where there is a high risk of death or where resources are severely lacking. Unpredictability refers to sudden changes in life, such as a parent leaving home or a lack of a reliable routine. In such a difficult environment, theory suggests that by reproducing early and often, a person can maximize their chances of passing on their genes before they risk dying early.
Previous studies exploring this theory have mostly relied on small groups of participants or specific surveys. Koehler and Rutherford wanted to see whether these patterns held true across the country’s population over time. They decided to look specifically at Canada to see how modern high-income environments affect large-scale reproductive schedules.
To test these ideas, researchers collected publicly available data from the 2006 and 2021 Canadian censuses. This 15-year gap allowed researchers to examine children’s environments in 2006 and observe reproductive outcomes for young people in 2021. The researchers looked at data at two different geographic scales to see which provided the most accurate predictions.
The initial geographic scale included 39,481 dissemination areas, which are small regions with approximately 400 to 700 people. The second size consists of 240 census divisions and is a much larger and more stable region made up of neighboring municipalities. The researchers hoped that larger regions would provide more reliable data because large-scale population movements across large national borders are less common.
Scientists extracted certain variables from the 2006 data that represent environmental harshness and unpredictability. The criteria looked at median household income, the percentage of homes in need of major repairs, and the percentage of households spending more than 30% of their income on rent. To measure unpredictability, they tracked unemployment rates, geographic mobility, and the prevalence of single-parent or divorced households.
For reproductive indicators in 2021, the authors looked at average family size, number of children per family, and average family size for single parents. They used statistical modeling techniques to see how well environmental factors in 2006 predicted reproductive outcomes in 2021. Scientists evaluated models based on how much variation in future reproductive patterns could be explained by past data. Variance simply refers to how much a number varies or spreads out between different regions.
As expected, the model using larger census divisions turned out to be the most accurate predictor. For these larger sectors, the 2006 data succeeded in explaining 81 percent of the variance in frequent reproduction in 2021. Historical data were also able to explain 64 percent of the variance in single-parent family size, a remarkable success rate for a psychological study.
The specific relationships they uncovered provided a combination of support and contradiction to psychosocial acceleration theory. Because of the high proportion of children living in low-income households in 2006, it was predicted that the number of single-parent households would increase even more in 15 years. This particular finding is consistent with the idea that early economic hardship influences future family structure and encourages faster reproduction.
However, other forms of difficulty predicted exactly the opposite of what the theory would expect. Higher unemployment rates and higher rents in 2006 were negatively correlated with both frequent reproduction and single parenting in 2021. In other words, areas with severe job insecurity and expensive housing will have fewer large families and fewer single parents in the future.
The authors also noted that standard measures of hardship may not capture the unique struggles faced by marginalized groups. In Canada, Indigenous peoples and visible minorities have historically experienced discrimination, systemic inequality, and trauma at disproportionate rates. The scientists suspected that these specific challenges may be acting as powerful environmental cues to promote reproduction, prompting them to include these demographic factors in their analysis.
“We also tested whether the proportion of visible minorities was a predictor of reproduction patterns,” Koehler told SciPost. “We know that visible minorities, such as indigenous peoples and immigrants, typically face much harsher and more unpredictable conditions than the general population, so we hypothesized that they were a predictor.”
“They turned out to be significant predictors, but much smaller than I personally expected,” he noted. “We were also surprised that in Canada, the proxy for hardship worked contrary to the PAT hypothesis; that is, places where people appeared to live a harsher lifestyle appeared to reproduce less frequently after 15 years. This also differs from the results we found in Brazil.”
To ensure they were looking at a timeline of human development rather than just a random statistical association, the researchers ran an inversion model. They tried to use 2021 data to predict 2006 results. This reversed timeline failed to yield an acceptable model that provides evidence that relationships flow in a particular direction of development over time.
“The average person would probably assume that the PAT postulate is true and would try to provide a resource-rich and predictable environment for their children,” Koehler says. “I think the concept of predictability is the biggest concept here.”
“In another study, participants were asked questions about their childhood, including whether their parents came home from work at about the same time each day, whether only one parent was home at any given time, whether meals were always served at the same time, and whether they knew their parents were supportive,” Koehler explained.
“It shows us that being there for our children every day can make a huge difference in many outcomes throughout their lives,” Koehler said. “I don’t think there’s any risk that the public will assume the PAT theory is correct even if it later turns out to be incorrect.”
As with all research, there are some caveats to consider. The most significant limitation is that it relies on geographic data rather than tracking individuals. There’s a good chance that the people who lived in a particular area in 2006 aren’t the exact same people who lived there in 2021.
“This study was conducted to test population data,” Koehler explained. “We were testing percentages or averages for census divisions and distribution areas, which can be understood as ‘census language’ for municipalities and groups of residences of about 10 blocks.”
Canada has high rates of immigration and internal migration, meaning changes in the population can influence the patterns researchers observe. Because of this geographic focus, this finding may not fully apply to individual human behavior. An area may show a mathematical link between early hardship and large family size, but entirely different groups of people within that area may be driving these two separate statistics.
“We were also collecting data in one year to predict data for the same region 15 years from now,” Koehler said. “We don’t know, and we can’t assume they’re the same people. So this is far from a definitive test of PAT or a claim that people living in the conditions described in the paper would have the reproductive patterns we found. Still, this finding should be useful for policy making and understanding population reproductive trends.”
Additionally, other explanations other than evolutionary theory may explain the results. Factors such as lack of institutional support, cultural differences, and local social norms significantly influence reproductive choices. In modern societies, access to higher education and health care tends to delay childbearing, which may explain why certain tough economic conditions actually predicted lower reproduction rates in this particular study.
In the future, Koehler plans to expand this area of research across different populations and formats. “We also tried research using social media data,” he said. “This theory is undergoing serious criticism and reexamination, so I think it would be helpful to collect data from as many sources as possible to see where and how the predictions are right and where they are wrong.”
“I will continue to do that if I have the opportunity,” Koehler added. “We test LHT-P predictions from multiple data sources (e.g. census, social media, university student participants, non-university samples) and from different countries with different levels of development and different cultures.”
The study, “Child Demographics and Socioeconomic Status Predicts Reproduction 15 Years Later,” was authored by Vinicius Betzel Köhler and Rutherford, MD.
