Socioeconomic status versus fitness
One common explanation for fertility declines over the last 200 years is that parents have shifted to investing in quality of children, rather than quantity. What is often not made clear is that this quality-quantity trade-off has two dimensions. The first trade-off relates to socioeconomic status (SES), with greater numbers of children resulting in less investment in education and resource dilution. The second trade-off relates to fitness, as a short-term increase in children may reduce fertility in future generations.
These two dimensions may not line up in modern settings. If an increased investment in quality increases both SES and fitness, there must be some point at which that investment pays off through more children. If not, despite the socioeconomic gains, the trade-off will not be delivering increased fitness.
A new article in the Proceedings of the Royal Society B: Biological Sciences by Goodman and colleagues presents some empirical evidence on this issue. Analysing a cohort of 14,000 Swedes born between 1915 and 1929 and their descendants, they showed that low fertility and high SES for parents increased child SES (as measured by school marks, university attendance and income) for as much as four generations into the future. However, this SES did not translate into higher fitness.
This result lends support to models which suggest that fertility is reduced for socioeconomic gains, but puts into doubt biologically based models in which reduced fertility is an adaptive trade-off for long-run fitness. The high SES people in the study had lower fertility than optimal, so the strategy pursued by these people is maladaptive.
One way in which SES affected fertility was the time between generations. While most of the low-SES people in the sample had four generations of children since the first study members were born, the higher SES members often had only three. In a growing population, you can have the same number of children per generation but lower fitness, as the shorter generation time of competitors allows them to increase in population more quickly.
The study did not analyse in-depth whether SES transmission is due to genetic inheritance, parental decisions to invest in quality over quantity, or resource transfers such as inheritance. As I am skeptical about the returns to investments in quality above a basic threshold (particularly in a developed country such as Sweden), I lean towards the genetic and resource transfer explanations.