Since the time of Cain and Abel, the birth order of siblings has been predictive of a few things — including one’s chances of pleasing the gods. Now, a new study suggests a child’s place in the family may influence health later in life.

A small study of middle-aged men in New Zealand found first-born children to weigh about 14 lbs. heavier, with larger measures of body mass index than second-born siblings. Moreover, the older siblings had higher insulin resistance, indicating a greater susceptibility to type 2 diabetes. Some evidence suggests birth order may also impact weight and metabolism beginning as early as infancy and into adolescence. But scientists aren’t sure whether birth order specifically impacts health midway through life. Investigator Wayne Cutfield led a study at the University of Auckland of 50 men in their 40s who were overweight but mostly healthy.

“Firstborn men were heavier and had lower insulin sensitivity than second-borns,” Cutfield wrote in the study, published this month in the journal Scientific Reports. “Thus, first-born adults may be at a greater risk of metabolic and cardiovascular diseases.”

Firstborn men had a 33 percent greater insulin insensitivity, on average. However, Cutfield emphasized that disease and health ailments may derive from many factors, with birth order varying in significance. "Being first born is one such risk factor, it does not mean first-borns will become overweight or diabetic, being first-born simply increases the risk."

Indeed, reviewers in Scientific American not long ago panned the design of research studying birth order, asserting that family size might matter more. “Put simply, birth order is intricately linked to family size,” wrote Joshua K. Hartshorne, a post-doctoral fellow at Massachusetts Institute of Technology. “A child from a two-kid family has a 50 percent chance of being a firstborn, whereas a child from a five-kid family has only a 20 percent chance of being a firstborn.”

Thus, statistics glorifying the high-achieving firstborn — think astronauts and statesmen — suggests family size might also matter, to some degree. That study design flaw affected most of the 65,000 scholarly articles on the subject of birth order, according to an estimate from Scientific American, but new research is showing a possible link to personality and intelligence. “Thus, the evidence seems to be shifting back in favor of our common intuition that our position in our family somehow affects who we become. The details, however, remain vague,” says Hartshorne.

Source: Albert B, de Bock M, Derraick J, et al. Among overweight middle-aged men, first-borns have lower insulin sensitivity than second-borns. Scientific Reports. 2014.