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A close look at the history of birth-order studies challenges recent reports that older sisters are more likely to be overweight than their younger sisters

Humble pie

Does being born first increase your risk of obesity?

By Alice Robb on September 3, 2015

A startling new study from the University of Auckland has been gathering attention in recent days, reported on internationally in newspapers and blogs. Drawing on nearly twenty years’ worth of medical records from the Swedish Birth Register and including 13,400 pairs of sisters, the researchers found that the first-borns were bigger babies and taller, heavier adults — 40 percent more likely to be obese and 29 percent more likely to be overweight than their younger sisters.

Wayne Cutfield, the lead author on the study, suggested that the differences are related to changes in the blood supply to the mother’s placenta between her first and second pregnancy; women’s blood vessels may be narrower during first pregnancies, restricting the flow of nutrients to the fetus and setting the baby up for less effective regulation of glucose and fat down the line. Or the differences could be environmental; parents might be more likely to indulge and overfeed their first child.

The paper was published in the Journal of Epidemiology and Community Health, and it’s been picked up this week by CBS, The Washington Post, The LA Times and others. Readers have jumped to use the findings to explain their own families and waistlines. “My older sister is, in fact overweight and has struggled with weight issues her entire life. On the other hand, it’s not been a problem for me, the youngest,” notes a commenter at Yahoo. “My Ex’s elder sister ballooned up…youngest sister of that clan stayed tall and trim,” writes another.

The study of birth order has a long and dubious history in psychology. Since the nineteenth century, birth order has been used to explain not just physical traits but personality, intelligence and even mate choice of adult siblings. Some of these ideas have become embedded in the popular imagination: that first children are bossy, that last children are immature. First children are said to have higher IQs and to score higher on measures of extroversion, agreeableness and conscientiousness than their siblings; last-borns have been found to be demanding, irresponsible and less intelligent.

But many of the early studies—the ones that launched the idea that birth order matters—didn’t control for important variables like family size and socioeconomic status. Better-off families tend to have fewer children, so a greater proportion of first-borns come from more affluent backgrounds—a much more important variable in life outcomes. As Scientific American points out, the fact that 21 of the first 23 astronauts to make it into space were firstborn— which is often trotted out as evidence that first-borns are especially intelligent and risk-taking—is more likely a reflection of their more comfortable upbringing.

Astronaut Sally Ride
Astronaut Sally Ride was an elder sibling.

Just last month, psychologists at the University of Illinois at Urbana-Champaign found that although there were consistent differences in IQ and personality across a wide sample of siblings, those differences were so small that they really didn’t matter. The study, which was reported in the Journal of Research in Personality, took into account 377,000 U.S. high school students and controlled for factors like family size and socioeconomic status. Although first-borns in this sample did have higher IQs, that advantage came down to a single point. And while that’s statistically significant, it’s imperceptible in real life. They also found, in keeping with other research, that first-borns were indeed more extroverted and agreeable— but Brent Roberts, who led the study, described the difference as “infinitesimally small.”

Though Roberts and his co-author, Rodica Damian, both praised the large sample size and methodology of the new Swedish sisters study, they cautioned against reading too much into it.

“Statistical significance is not a very good criterion for determining whether an effect is important,” said Roberts. “From what I can tell the effect appears to be very small.” The height difference in the adult sisters, for example, amounted to just about a millimeter. The study also reported that older sisters’ BMI was 2.4 percent higher during early pregnancy. “Does a 2.5 percent increase in BMI translate into meaningful health outcomes?” asked Roberts. He also pointed out that the focus on BMI could be misleading. “Waist circumference or direct measures of body fat would be preferable,” he said.

Overly deterministic interpretations (“Having trouble with your diet? Blame your siblings,” suggests Elle) can be not just pointless, but actually counterproductive, Damian warned.

“Just because we have this study, we should not look at weight in a deterministic fashion (e.g., ok, I’m a first-born, so this means I don’t need to exercise or eat healthy because I’m doomed anyway).”