Perhaps this study holds good for Norway, but not for the rest of the world. Different parts of the world account for differences in culture, expectation, values... Thus applying this study to the whole world is fundamentally flawed as the sample for the study was only taken from Norway. So yes this study may show that first borns in NORWAY have higher IQs than their younger siblings but it certainly does not show that first borns everywhere have a higher IQ.
Also, the use of the IQ measurement to discuss intelligence is rather questionable. Many psychologists doubt the effectiveness of this sort of test. Therefore again this study exceeds it's parameters in it's conclusions. It is not a study about intelligence but rather a study on IQ scores.
