In the comments below a reader asks about the empirical difference in heights between siblings. I went looking…and I have to say that the data isn’t that easy to find, people are more interested in the deeper inferences on can make from the resemblances than the descriptive first-order data itself. But here’s one source I found:
| Average difference | Identical twins | Identical twins raised apart | Full siblings |
| Height, inches | 0.67 | 0.71 | 1.8 |
| Weight, pounds | 4.2 | 9.9 | 10.4 |
| IQ | 5.9 | 8.2 | 9.8 |
These data indicate that IQ and height variation among sibling cohorts is on the order of ~2/3rd to 3/4th of the variation that one can find within the general population (my estimate of standard deviation of 2.5 inches for height below is about right, if a slight underestimate according to the latest data). But I also found a paper with more detailed statistics.
The aim of the paper was to find outliers from expectation. In other words, which siblings diverged a lot from what you’d expect in terms of normal variation within the cohort? In the process they do report some statistics on inter-sibling variation. The correlation of height between siblings after correcting for age and sex are 0.43. This is what I’ve seen in the literature. Next, the standard deviation is 6.7 centimeters. This is about ~2.7 inches. The average phenotypic difference between siblings was about 7.2 centimeters (D). Therefore, to a first approximation the recapitulation of population-wide variation in a continuous quantitative trait within sibling cohorts seems to hold. Though I’d be curious if readers can provide better and more diverse sources.