London, May 15 : Dreaming about being tall and well built like your older brother? Well, then you're in for a bit of disappointment, for according to a new study older brothers mean smaller siblings.
What's more: Having an older brother may reduce your chances of reproductive success.
Ian Rickard and colleagues at the University of Glasgow in Scotland collected data from 79 students and staff at their university with older siblings.
They found that those born with at least one older brother were on average lighter at birth and also shorter in adulthood than those born with one or more older sisters.
Rickard's team also analyzed birth records from kept by the Lutheran Church in Finland for 653 women between 1709 and 1815 and found that the lifetime reproductive success of those born with an elder sister was 27 percent higher than for those with an elder brother.
By including families in which the older son died before their sibling was born the team was able to determine that reduced reproductive success was not a result of interaction between siblings but due to some other factor.
The researchers suggest that mothers who produce sons perhaps incurred a reproductive cost that could have consequences for future offspring.
"Male fetal testosterone may cross over to the mother and impact her immune function, compromising the weight of future fetuses," New Scientist quoted Rickard, as saying.
"As birth weight and height are associated to health outcomes later in life, this could have potential implications to the long-term health of siblings," he added.
Low birth weight has been linked to ovulation disruption, increased risk of depression and lower IQ.
Because height is tied to attractiveness, there is some evidence that shorter men have fewer children. Having an older brother has even been linked with sexual preference.
The study is published in Evolution and Human Behavior.