Delayed childbearing among groups who carry genes linked with higher educational attainment may be causing these traits to become less common in some human populations, according to an Icelandic study. The study was published online January 17 in the Proceedings of the National Academy of Sciences.

Epidemiological studies have found that genetics may account for as much as 40% of a person's educational attainment, explained lead author Augustine Kong, PhD, a statistician at deCODE Genetics, a subsidiary of Amgen, based in Reykjavik, Iceland, which analyzes the human genome. Genome-wide association studies have revealed that some combinations of gene variations are linked to a greater likelihood of pursuing higher levels of education, he noted.

However, what might appear to be a genetic asset comes at a cost, in evolutionary terms. Studies in the United States and other countries have shown that individuals who stay in school longer have fewer children (Rindfuss RR et al. Demography. 1996;33:277-290).

To study the question further, Dr Kong and colleagues tapped a unique resource: a genealogical database that captures nearly every Icelander born since 1910. They generated a score for Icelanders who had been genotyped to determine how many educational-attainment linked genes they had. Then they analyzed whether having a higher score was linked to lower fertility in a population of 109,120 Icelanders, about one third of the island's total population.

"Using data from Iceland that include a substantial fraction of the population we show that individuals with high scores tend to have fewer children, mainly because they have children later in life," Dr Kong and colleagues write.

Women paid the highest fertility price. For each standard unit higher score, women had 0.084 fewer children, whereas men had 0.054 fewer children. Women's age at first childbirth also increased more than men with each higher standard unit score (0.59 vs 0.46).

The link between having a higher score and fewer children persisted even after the researchers adjusted for actual educational attainment. This suggested that the effect is not fully explained by individuals delaying childbearing while they were in school. The authors speculated that the score may be linked not only to cognitive ability but also to genetic traits that predispose people to "long-term planning and delayed gratification."

Between 1910 and 1990, this trend of later childbearing and fewer children resulted in a decrease in the average score for educational attainment-linked genes in a separate subset of 129,808 Icelanders. The decline in these educational attainment-linked genes was occurring at a pace of about 0.01 standard units on average per decade.

"In evolutionary time, this is a blink of an eye," write the authors. "However, if this trend persists over many centuries, the impact could be profound."

The study authors have disclosed no relevant financial relationships.

Proc Natl Acad Sci. Published online January 17, 2017. Abstract

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