Step Aside! Birth Rates on the Rise Among Taller, Thinner Women

Girls in Gambia
Taller, thinner women are giving birth to more children. Creative Commons

Using a unique data set involving thousands of women in rural Gambia, researchers from Durham University show how the trend towards living longer and having fewer children may also lead women to be taller and slimmer.

"This is a reminder that declines in mortality rates do not necessarily mean that evolution stops, but that it changes," said Dr. Ian Rickard, a lecturer in the Department of Anthropology.

The UK Medical Research Council collected data over a 55-year period (1956-2010) on thousands of women from two rural villages in the West Kiang district of Gambia, including survival, fertility, and anthropometric data. Over the time period, those communities shifted from high mortality and fertility rates to rapidly declining ones.

Meanwhile, analysis of the data showed that selection initially favored short and heavier women. By the end of the period, though, this preference had reversed with taller, thin women having more children than average.

The researchers believe that better healthcare — a clinic opened there in 1974 — may have influenced which women were more likely to reproduce.

Although they were unable to determine precisely the reasons behind the evolutionary shift, researchers say their findings in Gambia may have relevance around the globe.

"The majority of human populations have either recently undergone, or are currently undergoing, a demographic transition from high to low fertility and mortality rates," said Rickard. "Therefore the temporal dynamics of the evolutionary processes revealed here may reflect the shifts in evolutionary pressures being experienced by human societies generally."

Evidence exists for a similar trend in Great Britain. Separate studies found that taller men and women with lower than average weights tended to have more children.

Results for the study in Gambia were published today in Current Biology.

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