Limiting the number of children can increase wealth but decreases evolutionary success.

A general observation is that people in the developed countries have fewer children. This is the opposite of what is expected because having more resources would mean that people could reproduce more. However, increasing wealth often leads to low fertility rates with people deliberately trying to produce less, a phenomenon known as "demographic transition." In developing countries, however, having a large income leads to larger family.

An "adaptive" hypothesis has been put forward to explain the phenomenon of demographic transition. According to the theory, having low fertility rates and conserving wealth actually advances evolution because the subsequent generation would be able to produce more children given their socioeconomic success.

For the study, researchers studied 14,000 people born in Sweden during 1900s and their descendants being tracked under the Uppsala Multigenerational Birth Cohort Study. 

The present study found that the socioeconomic success shows an upward trend for up to four generations, but reduced the number of children in later generations. The choice for having fewer children is an economic strategy and does not benefit evolution.

The research shows that a person's education level and their income affects the education and income levels of not only their children and grandchildren but their great grandchildren as well, according to Professor Ilona Koupil, from the Centre for Health Equity Studies (Stockholm University/Karolinska Institutet).

Researchers assessed socioeconomic success of each generation by analyzing school records, the number of college degrees and evolutionary success by age at marriage and number of children.

The found that smaller families indicated that the parents were wealthy. Children in these families did better at school, had college degrees and passed on the family wealth to the next generation. Despite the advantages, there were less descendants, contrary to the "adaptive" theory put forth by researchers.

"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," said lead author Dr. Anna Goodman, Research Fellow at the London School of Hygiene & Tropical Medicine.

Goodman added that having more children would mean that the children will be less likely to reproduce or that a higher quantity will result in low quality. Researchers found this to be true in socioeconomic status of successive generations and not in their biological success.

"One of our most interesting findings is that being from an initially wealthy household makes the benefits of small family size even bigger. Poorer households in contrast have relatively little to gain by limiting fertility, perhaps because the success of their children is more determined by broader societal factors, rather than investment and inheritance from parents, which is in short supply," said Co-author Dr. David Lawson, from the Department of Anthropology at UCL.

The study was published in the journal Proceedings of the Royal Society B: Biological Sciences.