Small family size increases the wealth of descendants but reduces evolutionary success

As societies get richer, family size generally falls. This puzzles evolutionary biologists, because natural selection usually favours organisms that maximize opportunities for reproduction. Research published in Proceedings of the Royal Society B this week, reveals that having a small number of children increases the social and economic success of descendants across up to four generations, but reduces the total number of long-term descendants. 

Researchers from the London School of Hygiene & Tropical Medicine reject the popular ‘adaptive’ hypothesis, which proposes that low fertility may boost evolutionary success in the long term by increasing offspring wealth, which in turn eventually increases the number of long-term descendants because richer offspring end up having more children.  

The researchers in fact conclude that the decision to limit family size can be understood as a strategic choice to improve the socioeconomic success of children and grandchildren in modern societies, but this socioeconomic benefit does not translate into an evolutionary benefit.

The study indicates a conflict in modern societies between behaviours promoting social and economic benefits versus biological success. This contrasts with traditional populations in the developing world, where behaviours that promote wealth and social status usually lead individuals to leave behind more genetic descendants.

The researchers tested these hypotheses using data from the Uppsala Multigenerational Birth Cohort Study, which tracks 14,000 people born in Sweden in the early 1900s and all their descendants to the present day.

They measured the socioeconomic success of each generation by looking at their school marks, at whether they went to university and at their household income across adulthood. Reproductive success was assessed by survival to adulthood, marriage before age 40 (a proxy for ‘mating success’) and fertility (number of offspring up to 2009).

Among both male and female children in the original cohort, smaller family size and higher parental socioeconomic position were both associated with substantially higher school marks, university entrance and income.  These effects were particularly large when low fertility and high socioeconomic status coincided, with the benefits of small family size being particularly marked in wealthier groups.  Moreover, these advantages were in turn passed on to the grandchild and great-grandchild generations.

But contrary to the adaptive hypothesis, small family size and high parental wealth either did not affect reproductive success beyond the first generation of offspring or, if anything, showed a negative effect in subsequent generations.

Lead author Dr Anna Goodman, from the London School of Hygiene & Tropical Medicine, said: “Under natural selection, you would expect organisms to use their resources to produce more genetic descendants, and so increase their Darwinian fitness.  The demographic transition is a puzzle because at first sight it doesn’t look like people are doing this.  One adaptive explanation for the puzzle is that there exists a quantity-quality trade-off, such that having more children leads to those children being less able to reproduce in turn – i.e. higher ‘quantity’ leads to lower biological ‘quality’.  However our study found this quantity-quality trade off only applied to descendants’ socioeconomic success, not their reproductive success.”

Co-author Professor Ilona Koupil, from the Centre for Health Equity Studies (Stockholm) said: “It is important to note the equity implications of these findings.  First, this research indicates that differences in family size may have lasting consequences on social inequalities.  Second, this research provides evidence for the fact that people’s educational levels and wealth not only affect schoolmarks and income in their children but also in their grandchildren and great-grandchildren.  From a broader social policy perspective, our findings show that even in a country like Sweden with relatively low levels of inequality, we need policies that seek to equalise children’s opportunities across families.”