Scientists in Norway have confirmed after carrying out a large study that the order people are born in within a family may influence how smart they are.
The possibility of a connection between birth order and IQ levels is a factor which has been suspected for some time but until now the reasons were unclear.
According to a team at the University of Oslo, the eldest child is normally the one with the highest IQ and this they believe is the result of family up bringing rather than any biological difference.
In their study the researchers used data on birth order, status of earlier born siblings and whether they had died early in infancy, and IQ scores for nearly 250,000 male 18 and 19 year old Norwegian military conscripts who were drafted between 1985 and 2004.
They found on average around a 3 points higher IQ in the eldest brother when compared to the second son, and an average 1.1 point higher level in the second son when compared to the third.
Similar findings on the same subject, made in earlier studies were criticised on the basis that the families were all in different situations and the results were as a result subjective.
To avoid issues of biological effect and the differences complained about before Kristensen and his colleagues looked at brothers raised in the same families who had had lost their first born or first and/or second born children in the first year of their lives.
They say this demonstrated that it was how the boys were raised rather than the biological factor in birth order that made the difference.