Big brother has a lot to answer for

December 07, 1999

HERE'S something else boys can blame their older brothers for: a slightly skewed body shape. So say doctors in Ontario, who have shown that the more older brothers a man has, the more likely he is to show random asymmetries in his body shape, a sign of troubled development in the womb.

Martin Lalumire of the Centre for Addiction and Mental Health in Toronto and his colleagues studied 40 adult male psychiatric patients and 31 male hospital employees. They were asked about how many older and younger siblings they had, and whether they were brothers or sisters.

They also looked for random differences between one side of the body and the other. If our development in the womb were flawless, we would be perfectly symmetrical. The small asymmetries we have are thought to be the result of the problems faced by the developing fetus, such as a mild infection in the mother.

The team measured features such as finger lengths and ear size. Some variations between left and right were excluded because they may have a function. People are most often right-handed and seem to have more ridges on the right hand than the left, which possibly helps their grip.

But five asymmetries did seem to be random: hand and ear width, and the lengths of the ear, third and fifth finger. Having an older brother predisposed an individual to be less symmetrical. The more older brothers, the more marked the asymmetry was likely to be. Older sisters had no effect.

The researchers speculate that male fetuses trigger an immune reaction in their mothers. Genes on the Y chromosome, which is unique to males, produce an antigen called HY, and this may set off the mother's alarm bells. Each successive male would boost that response. This theory is backed indirectly by other studies. One showed that the placenta, which protects a fetus from the mother's immune system, increases in size with each successive male.

The good news is that brothers pose no risk to girls. "Even though mum's immune system is primed, there's nothing to respond to," says Lalumire.
Author: Alison Motluk, Toronto

Source: Proceedings of the Royal Society B (vol 266, p 2351)


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