Temperature, sunshine, seasonal foods, winter infections: All can affect… (Top and bottom right: Brian…)
If you don't believe in horoscopes, you're in step with science. But that's not the same as saying the season of your birth cannot affect your fate. Hundreds of studies, published in peer-reviewed journals, have suggested that the month a person is born in is associated with characteristics such as temperament, longevity and susceptibility to certain diseases.
Scientists say that even though some of these findings are probably spurious — if you dig around in data, you will eventually find correlations just by chance — other effects are very likely real, triggered not by the alignment of the planets but by exposures during prenatal and early postnatal lives.
Temperature, sunshine, seasonal foods, winter infections: All can affect the development of a baby and, as a result, its future. Here's a look at the findings and what scientists make of them.
Winter (Dec. 22 to March 20):
Body weight: Men with a few pounds to lose may have another excuse why that stubborn fat won't leave. David Phillips of the University of Southampton in England analyzed the weight of 1,750 British seniors and found that 13.8% of men with birthdays from January to March qualified as obese but only 9.4% of those born from October to December did so. In women, seasonal trends were less apparent. Phillips argues that exposure to low temperatures in early life might promote development of fatty tissue and predispose winter-borns to obesity in adulthood. Lab studies support his theory: Rats subjected to cold before or soon after birth store more ingested energy as fat, even when the temperature is no longer low.
Temperament: A 2004 psychological questionnaire of 448 men and women found that 20- to 45-year-olds born during the half year containing winter (October to March) are more likely to be novelty seekers — curious, hating monotony, apt to choose sky diving over Sudoku as a hobby. The same didn't apply to those older than 45: At these ages, winter-borns were less interested in novelty-seeking than summer-borns, suggesting they settle down faster. "Season of birth does influence temperament; we just don't know exactly why," says Lars-Göran Nilsson, a psychologist at Stockholm University in Sweden, who in 2001 published a study on this subject in the journal Neuropsychobiology. He thinks it has something to do with levels of serotonin and dopamine, key brain chemicals that seem to be involved in formation of personality. "Throughout the year, their production fluctuates in a mother's body and might affect development of a fetus," Nilsson says. Animal studies suggest the fluctuations are due to changes in day length.
Schizophrenia: The rate of this severe mental illness in the general population is about 1%. But those born in winter develop it at higher rates, an observation that dates to 1929 and has since been confirmed in more than 200 studies. One of these, published in 1999 in the New England Journal of Medicine, found that people born in the Northern Hemisphere in early March were 11% more likely to develop schizophrenia than those born in early June or early December. South of the equator, the pattern is reversed. A popular hypothesis blames prenatal infections, with some researchers pointing fingers at the seasonal flu and others at the summer and autumn epidemics of rubella, polio and diphtheria that could harm the fetus during the first trimester of pregnancy. People born in August and September have the lowest risk of schizophrenia.
Spring (March 21 to June 21)
Height: People born in spring tend to be taller, according to Gerhard Weber, an anthropologist at the University of Vienna. Analyzing Austrian Federal Army measurements of more than half a million men, he found that the tallest recruits were born in April, the shortest in October. Although the average difference was small (0.2 inch), the finding, reported in 1998 in the journal Nature, was statistically significant. Weber speculated in his study that the effect could be due to concentrations of the light-dependent hormone melatonin in the mother's body, which might stimulate secretion of growth hormones. Since human fetuses and babies grow in spurts, the amount of daylight during periods of fast development would be crucial.
Multiple sclerosis: Spring-borns have an elevated risk of this inflammatory disease, known to be caused by a combination of genetic and environmental factors. Studies have established that the higher the latitude of a country, the higher the prevalence of MS. How big a rise? Among Australian states, the risk in temperate Tasmania is fivefold that in subtropical Queensland, and a 2005 study of 17,874 Canadian and 11,502 British patients with MS found that those born in May have a 23% increased risk of developing MS compared with those born in November. The suspected cause: deficiency in maternal concentrations of vitamin D at the end of the second or third trimester.
Summer (June 22 to Sept. 22)