3 IQ points, that is. The statistically significant, but not quite practically, significant difference in the IQs between eldest Norwegian men and their siblings. Jake Young says so what here.
To put this into perspective, the traditional standard deviation of IQ is 15 points. In the standard deviation for height in inches in the USA for 19 year olds, is about 5 inches. So, 3 IQ points can be compared to to one inch of height for American 19 year olds.