For people in the Bolivian Amazon, having a younger brother is tied with higher blood pressure.
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Having a younger brother isn't just a drag in childhood, it may also raise your blood pressure into adulthood, suggests a new study on Bolivian adults.
The effect, detailed this month in the journal Economics and Human Biology, is only correlational and so the scientists can't say whether one causes the other. But they do offer a mechanism, suggesting that in this region, a younger sibling — particularly a younger brother — takes parental attention from older children while burdening the older kids with extra responsibility.
"If there are more siblings in the family, the parents' resources are limited. The older brother will see the younger brother as a competitor and it will kind of stress them out," Zeng said.
In wealthy, developed countries, the same link would likely not hold and may even go in the opposite direction, other research has suggested. [10 Scientific Tips for Raising Happy Kids]
Sizing up siblings
It's been well documented that birth order can affect personality, sexual maturation and several economic outcomes. In a previous study, Zeng's team found that adults with younger siblings tended to have higher total cholesterol than other children.
To see whether blood pressure was affected, too, Zeng and his colleagues measured the blood pressure of 374 adults living in 13 Bolivian villages in the Amazon. Many came from large broods of six or seven children, so the group included a mix of younger, older and middle children.
Younger brothers seemed to bring the biggest health risk: Those adults with younger brothers had blood pressures up to 6 percent higher than others in the study. Younger sisters were also a problem, but only for older girls, whose blood pressure increased by 3.8 percent for each younger sister.
But the blood pressure spike probably wasn't caused by sibling spats or by younger brothers' behaviors. Rather, having younger kids in the family may have drained parental attention. And looking out for the younger siblings requires a lot of work from elder children.
"In that area the older brothers have responsibility to help the younger brothers to find a job in the future or to find a wife or girlfriend," Zeng told LiveScience.
Perhaps because males are preferred over females in the region, baby sisters require fewer resources than baby brothers and so weren't tied to a jump in blood pressure as much, Zeng said.
Luckily, the stress of younger siblings wanes as people age.
"When people get older, they're not living with their brothers or sisters anymore," he said. They have more control over their lives and fewer burdens from younger sibs, he added.
People in developed countries can't blame younger brothers for their soaring blood pressure. With fewer siblings, many more resources, and fewer responsibilities to younger brothers, siblings in rich countries may actually lower blood pressure, according to a 1991 study in the New England Journal of Medicine.
Instead, Americans can thank their sedentary, calorie-rich lifestyle for high blood pressure, Zeng said.