Younger siblings and people from larger families are more likely to suffer from cardiac events such as strokes or heart attacks, new research has found.
A peer-reviewed paper published by BMJ Open examined the results of a large population study in Sweden and cross-referenced them with figures on cardiovascular events in patients.
Firstborns were shown to have better cardiovascular health outcomes in later life, and those from smaller family units were more likely to be healthier than people in large families, it showed.
Prof Peter Nilsson, from Lund University in Sweden and one of the study's authors, said the findings showed a clear benefit for firstborns.
He said he was surprised that the data showed a small but "robust" difference in cardiovascular disease.
Data for 1.36 million men and 1.32 million women born between 1932 and 1960 and aged between 30 and 58 was analysed.
"Firstborn have more parental attention and supervision. This is why I think they tend to conform and behave well, including less smoking and alcohol overuse", Prof Nilsson told The National.
The data showed men with at least three or four siblings had worse outcomes for cardiovascular health than those from a smaller-sized family with one sibling. The figures remained broadly similar for women.
However, children with no brothers or sisters suffered from more health problems than those from small families.
Firstborns had a slightly higher risk of death, which could be partly explained by the risks posed to first-time mothers, Prof Nilsson said.
"Firstborn may be more prone to die at delivery or soon thereafter because the mother is not used to delivering and raising a child."
Socioeconomic factors could explain these worse health outcomes. Poorer families in the West tend to have many children and this may have a negative effect on siblings born later.
"The number of children in a family is socially patterned, but some wealthy and some poor families tend to have many children," he said. "Religion can also play a part."
However, later-born siblings might have an advantage to learn to speak more rapidly than their precursors, he said.