Oct. 25 2016

The Curious Link Between Family Size and Height

Sons born to large families were nearly an inch shorter than those from small families, according to a study of Dutch military records

If you find yourself staying in an old bed and breakfast in Holland, you may have to duck your head when walking through the door and sleep with your feet hanging over the end of the bed. This is because over the past 150 years, the average height of Dutch men has increased from about 5-foot-4 to over 6 feet, and they are now among the tallest people in the world.

There have been similar height increases in other European countries and the United States. And while most of a person’s stature can be attributed to genetics, some researchers estimate that a fifth of height can be attributed to environmental factors like the availability of food and social resources.

With this knowledge, Laura Stradford, MPH ‘15, examined data on uniform measurements for Dutch military conscripts as part of her Mailman School Epidemiology thesis and found that men with more siblings were shorter than those born into smaller families. Under the advisement of L.H. Lumey, the results of her work were recently published in the journal, The History of the Family.

According to Stradford, the link between family size and height is likely explained by what is known as resource dilution. As families grow, parents have less to give to each child.

“If you have a small family, maybe you have more time to read to the children and to interact with them. Maybe you have more money to buy food and medical care,” Stradford explains. “Whether you’re rich or poor, if you’re from a larger family you’re likely to be a bit shorter.”

There was a difference of almost an inch between the height averages of families with one child compared to nine or more, which was seen in both high and low socioeconomic statuses. Recruits from relatively more affulent backgrounds were significantly taller. The study finds new evidence that the men were taller than their younger brothers —although the effect was less pronounced than family size.

The study used height and demographic information from 389,287 male military recruits in the Netherlands, age 18, from 1962-65. Because service was compulsory during this period, the data is representative a broad spectrum of socioeconomic and demographic groups.

Some of the men were born around the time of the Dutch famine of 1944-45, but previous work done by former Mailman professors Zena Stein and Mervyn Susser found that the famine had no effect on the recruits’ heights when experienced during birth. Instead, adult stature was most determined by the growth spurt in late adolescence.

In examining their findings, the researchers looked at a number of alternate explanations, including whether religion made a difference, since Roman Catholic families were known to have had larger families than the general population during the period studied. After adjusting for religion, however, the results held up.

While family size has trended smaller since World War II, the researchers say the shift happened slowly enough that it wouldn’t skew the results. And while parents who have larger families would have been aware that having more children means there is less to spend for each child, the researchers say it is unlikely that parents anticipated that this would affect the height of their offspring and adjusted their family size decisions accordingly.

While stunting can happen as the consequence of disease, poor health is far from the main determinant of height. For that reason, the findings shouldn’t be interpreted as meaning that larger families or shorter people are less healthy, cautions Lumey, the study’s senior author.

“At the population level,” he says, “height is a general indicator of how equitable societal resources are spread across the socio-economic strata. It is an important marker of socio-economic inequality.” And time and again, socio-economic status has been shown to be one of the clearest indicators of health.

An unanswered question is whether the connection between family size and health also applies to women vis-à-vis their sisters and their brothers. According to Lumey, one possibility would be to examine data from military recruits in Israel where everyone, men and women, are required to serve.