Researchers are taking a fresh look at the global decline in birth rates, focusing not only on economics but also on evolution and urban life. Even countries that once saw rapid population growth are now facing ageing societies and fewer young people.
A large-scale study published in Archives of Sexual Behavior analyzed more than 4,400,000 survey responses from men and women across 23 countries. The scientists set out to understand how where people live and how much they earn influence the number of children they have.
Population density and family size
One of the clearest findings was that higher population density is linked to fewer children. It is not simply about living in a big city, but about how many people share the same limited space and resources.
When many families compete for housing, jobs, green spaces and services, the cost of raising children rises sharply. In such conditions, parents often prefer to invest heavily in one or two children rather than trying to support a large family.
This pattern aligns with life history theory in evolutionary biology, which describes how species allocate energy between survival, growth and reproduction. Humans are a species that tends to favor fewer offspring, but with intensive care and investment in each child.
How income changes the equation
The study’s most intriguing result is that income can soften the impact of high population density on fertility. In crowded urban areas, wealthier respondents tended to have more children than those with lower incomes.
According to the authors, financial resources help offset the difficulties of city life, from expensive housing and education to access to childcare and domestic help. For people with stable, high incomes, living in a megacity does not necessarily force a choice between comfort and a larger family.
This financial buffer can provide better schools, safer neighborhoods and more flexible work arrangements. As a result, affluent families may feel more confident about having additional children even in highly competitive urban environments.
The so-called Musk effect
The link between wealth and family size appeared stronger for men than for women. Researchers described this pattern as the “Musk effect,” referencing Elon Musk as a prominent example of a very wealthy man with many children in an urban, high-tech world.
Similar cases can be observed among some political and business elites, where men with significant resources maintain large families despite intensive careers. The analysis suggests these examples reflect a broader trend rather than isolated exceptions.
This raises questions about how social status, success and parenthood are connected for men and women. It may also highlight persistent gender norms that still shape expectations around breadwinning and family planning.
What this means for policy and people
The authors stress that population density does not rigidly determine how many children any individual couple will have. Choices about parenthood are also shaped by culture, health, personal values, relationships and housing conditions.
However, the findings help explain why fertility tends to fall in large, expensive cities and why financial security can counteract that decline. For governments, this offers arguments in favor of targeted support for families.
Policies such as affordable housing, accessible childcare, parental leave and well-designed urban planning could ease pressure on parents. By reducing the trade-off between living in a big city and raising children, countries may be better able to address long-term demographic challenges.
For individuals, the research is a reminder of how powerfully our environment influences life decisions that often feel deeply personal. The city around us, and the resources we have within it, quietly shape what we imagine a family can be.
