Big brothers blamed again

Behind the Headlines

Thursday November 20 2008

Big brothers always have a lot to answer for

Having a big brother “cuts your chances of children” reports the Daily Mail today. It said that researchers believed the fertility of younger brothers or sisters could be affected by having an elder brother. The newspaper added that the phenomenon is thought to begin in the womb as “the physical cost of carrying a boy may take so much out of a woman that the health of her next child suffers”.

The story is based on a study that found that people with an older sister had a 67% chance of having children, compared to only 62% in people with an older brother.

However, the study was of three generations of Finnish people in the 18th and 19th century. There are many differences between present day UK and pre-industrial Finland, where there was a low life expectancy (23 years), high rates of death in childhood (only about half survived beyond 15 years), a relatively late age of becoming a father (26.7 years) and strict social monogamy (97% of couples having children were married).

Although the researchers say that their findings support a biological explanation, it seems equally possible that the slight difference in the chance of having children is due to social reasons, such as the bias shown towards a firstborn male child.

Where did the story come from?

The research was conducted by Dr Ian J. Rickard and colleagues from the Department of Animal and Plant Sciences at the University of Sheffield. The work was funded by the Natural Environmental Research Council, The Royal Society of London and The Academy of Finland. The study was published in the (peer-reviewed) science journal: Evolution and Human Behaviour.

What kind of scientific study was this?

In this cross-sectional study, the researchers analysed data from population registers maintained by the Lutheran church in the 18th and 19th centuries in pre-industrial Finnish farming and fishing communities.

The researchers state that previous research in animals and humans has shown that siblings can have an effect on an individual’s survival and reproductive success. In humans, this interaction is also affected by the sex and the number of siblings that an individual has. They say social explanations are usually given for this effect, such as parents’ bias towards firstborn sons.

The researchers aimed to investigate if there are potential biological reasons for this effect. They wanted to see if the sex (male or female) of a firstborn child could have an effect on the health of their younger brothers and sisters. They reasoned that if there was an effect, and it was irrespective of the younger child’s sex, it would be evidence of a biological explanation.

Using data from five farming/fishing communities in Finland, the researchers collected data on the survival and reproductive events of 653 fertile women born between 1709–1815. They also recorded the full life history data for their 4,515 children as well as the birth and childhood mortality of their 7,846 grandchildren. Data on social class, parish birth order and family size were all available. Complex statistical techniques were used to adjust the data and to test for the statistical significance of their results.

What were the results of the study?

A biological explanation for the observed difference is not any more plausible than a social one.

About 65% of the individuals who survived to adulthood had at least one child. The probability of having a child varied between communities and poorer people had relatively fewer children than those who were rich or middle-class. The probability of having children also declined with increasing birth order, meaning that younger brothers and sisters were less likely to reproduce than older ones.

The main finding is that (when these two previous associations were taken into account) individuals whose mothers had previously had a son, experienced a significant 5% reduction in the probability of reproducing compared to those whose mothers had had daughters first. This was given as 67% for siblings with older sisters compared to 62% for siblings with older brothers. These findings were not affected by whether the younger sibling was male or female. There was also a longer gap between births when the firstborn was male compared to when they were female.

What interpretations did the researchers draw from these results?

The researchers say their research highlights that differences in the success of offspring are not always social in origin, but can also be biological. This is caused by the different [biological] costs for mothers of producing sons versus daughters.

What does the NHS Knowledge Service make of this study?

This study found that when the firstborn in a family was a boy, the next child was slightly less likely to have children of their own compared to when the firstborn was a girl. The researchers propose that there is a biological rather than social or cultural explanation for this.

However, the results are not strong enough to prove this theory or make it any more plausible. The researchers themselves call for more studies in populations of humans and animals in the wild to see if the findings can be repeated. It is too soon to conclude that a biological mechanism is the sole explanation for the small difference found here.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Having a big brother 'cuts your chances of children'. Daily Mail, November 19 2008

Links to the science

Rickard IJ, Lummaa V, Russell AF. Elder brothers affect the life history of younger siblings in preindustrial humans: Social consequence or biological cost? Evolution and human behaviour 2008; Published online 01 October 2008

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