Boys and girls do NOT run in the family and the chances of having a child of either sex is genuinely random, scientists say
- It is often thought that the ratio of boys/girls in one’s offspring is a heritable trait
- However, experts studying the sex of people born in Sweden found the opposite
- You are essentially as likely to have a boy as a girl, regardless of past generations
The tendency to have boys or girls does not run in the family and the chances of having a child of either sex is genuinely random, scientists have found.
Researchers studied the sex of people born in Sweden over the last 90 years to determine whether or not people are more likely to have children of a given sex.
Their findings shake up key theories over sex ratios used by biologists, who often assumed that the ratio of boys to girls in one’s offspring was a heritable trait.
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Pictured: in the Brady Bunch, a lovely lady with three very lovely girls met a man with three boys of his own. In real life, however, the tendency to have boys or girls does not run in the family and the chances of having a child of a given sex is random, scientists report
The ratio of boys to girls among one’s children — which researchers refer to as the ‘offspring sex ratio’ — is known to be around one-to-one.
Experts explain this in terms of the so-called Fisher’s principle, which says that — in a population with an uneven ratio of men to women — individuals of the rarer sex will be more valuable from a reproductive perspective.
Accordingly, the theory goes, individuals who are genetically disposed to have more offspring of the rarer sex will be more successful, so more children of the rarer sex will be born until the population ratio evens out.
Fisher’s principle relies, however, on offspring sex ratio being a heritable property that can be passed down the generations — but this has not been proven.
In their study, Dr Zietsch and colleagues analysed the entire population of people born in Sweden from 1932 to the present day.
This included 3,543,243 individuals and their 4,753,269 children.
The team looked to see whether there was any association between the sex of an individual’s offspring and the sex of their siblings’ offspring.
The researchers found that the ratio of boys to girls a person has is not an inherent property that passes down through the generations. Instead, the sex of one’s children is essentially random, although with slightly more male children born in general
The researchers found that the ratio of boys to girls a person has is not an inherent property that passes down through the generations.
Instead, the sex of one’s children is essentially random, although with slightly more male children born in general.
‘We estimated that the heritability for offspring sex ratio was zero […], rendering Fisher’s principle and several other existing hypotheses untenable as frameworks for understanding human offspring sex ratio,’ the researchers wrote.
The full findings of the study were published in the journal Proceedings of the Royal Society B: Biological Sciences.
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