It's a boy, not an old wives' tale
Old wives' tales abound for influencing or predicting the sex of a baby. If the myths are to be believed, hopeful parents should be moving their bed to face a particular compass point, timing the precise point of ovulation, judging the shape of the growing 'bump' or studying swinging wedding rings suspended over the expectant mother's palm. A report in the 25 September 1997 issue of the science magazine Nature tells how older husbands might be more reliable predictors of the sex of a child than are old wives.
To be precise, it is the difference between the ages of the parents that seems to have a small but significant effect on the sex of their first child. The effect was discovered by Dr John T. Manning and his colleagues from the Population Biology Research Group of the University of Liverpool, UK, within historical population records from England and Wales. The bigger the parental age difference, the more biased the child sex ratio, with comparatively older fathers producing more boys.
Their results stem from a noticeable change in marriage and birth patterns around wartime. Manning and his team say that "The proportion of male to female births increases during and shortly after periods of war", with male births rising to around than 51.5 per cent around the First and Second World Wars. They also found that the average age difference between spouses changed in favour of older husbands around those times.
Using statistical tests they linked the two observations, claiming that the age difference between spouses was strongly correlated with the sex ratio between the years 1911 and 1952. The bigger the age difference between parents, the stronger the effect. Looking at the first- born child, the researchers found an excess of daughters when the mother was older than the father and an excess of sons when the father was older than the mother. The latter category is more noticeable because that parental age bias was the more common around war time. To some extent, the second-born child restores the balance, with a very small excess of daughters for older fathers and younger mothers, although this effect was much less prominent than the bias of the first-born child, and did not overcome the general trend towards more boy babies.
But statistics yields only trends, not reasons. Why should the sex bias occur? Intuitively it seems natural to increase the birth rate of boys when the male population was depleted during the two World Wars. In general, there is a small bias towards male births which may compensate for the slightly lower male life-expectancy. But the mechanisms that cause the bias are a mystery.
Rank and dominance in some animals is related to the sex of offspring. The researchers suggest that the bias may be the result of women preferring to marry older men with higher resources. Perhaps the hormonal or other influences of rank and status might also influence early miscarriage or the implant rates of embryos of a particular sex. Another possibility is that somehow the mother might influence the movement of sperm carrying the male (Y) or female (X) chromosome.
But before anyone goes to trade their partner for one of the appropriate age, be warned. The effect is small and noticeable only on the scale of whole populations, not individual cases. That swinging wedding band may turn out to be a more reliable guide. By the way, if it swings from side-to-side, it's a boy, and circling means a girl -- according to old wives.