New Scientist has published a piece on the relative IQs of children, noting that the first child is more likely to have an IQ that is higher than his/her younger siblings.
Keeping in mind that I am a younger sibling in my own family, I have to admit in my own experience the IQ situation agrees with the findings of this research. Nevertheless, as has been noted over and over, a higher IQ is no clear indicator of success in life. A high IQ may be a better indicator of financial success than socio-economic factors, but there is ample evidence that under-achievement can often be characteristic of intellectually gifted children.
What’s actually interesting about this study is that it has identified social factors as a leading factor in IQ results. For decades it has been known that IQ tests are generally culturally biased (so if you are regarded as a native speaker and have been brought up in the region in which you take an IQ test your results are likely to be higher than those who are newer citizens of a culture), but now there is evidence that the effects of social environments on IQ are far more profound than initially indicated. What would be interesting is if the lower IQ of younger siblings could also be correlated with a higher EQ.