Why do some women only give birth to boys or to girls?

BACKGROUND:

There are more families with many children of only one sex than can be explained by unconditional probability.

MATERIAL AND METHODS:

Data from the Medical Birth Registry of Norway, covering more than 540,000 women with two, three and four births for the period 1967-2003, were used to study how the sex distributions of children already born affected the probability of a new birth and the sex of the next sibling.

RESULTS:

Women with two children of the same sex had a higher probability of having more children compared to women with two children of both sexes (RR=1.14, (1.14-1.15)). This also applied to mothers with three children (RR=1.15 (1.13-1.17)). The probability was highest for mothers with boys only. Multiple births and parity affected the probability of giving birth to a boy versus a girl, but the sex composition of already born siblings had no influence.

INTERPRETATION:

We have no evidence that some parents may have a probability of having a boy versus a girl that differs from the rest of the population. The fact that there are more siblings of only one sex is mainly behavioural; some mothers with only girls or only boys keep having more children in an attempt to receive a child of the opposite sex.

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