Smarter older siblings

The study, published in Science, has conducted Petter Kristensen , University of Oslo and Tor Bjerkedal of the Norwegian armed forces. The study consists of data on a colossal to 241.310 men between 18 and 19 years used statistical sample .

It is based on intelligence tests IQ that were used in Norway to enter military service in the sixties and seventies of last century.
As results, on average , older siblings had an IQ of 103.2 , 100.4 while the second brother . The third would be almost the same as the second at 99 . Thus a relationship between birth order and intelligence is established.
Birth order is not the key to all this , since it is sufficient that the smarter brother has served as a big brother but biologically not. Naturally this is an average and there are exceptions to the rule more intelligent than older siblings .
It was thought that studies on the subject, claiming the same result , were contaminated by uncontrollable artifacts such as family size , socioeconomic factors , status, parental intelligence , etc. . He even went on to show that younger siblings have better physical health and constitution that older , so the thing would be reversed from a biological standpoint .
But this study is so exquisite that you can no longer doubt the results .
The reason behind this fact is unclear. Input seems that the cause is more social than biological ( or genetic or pregnancy related ) .
It has been hypothesized that older siblings have to tutors younger and that this function or task that would improve their intelligence .
Thus a second brother to which is the older brother dies must assume the task would be entrusted to the greater of instructing young and eventually end up being the smartest reaching the same level reached by his older brother died .