For comparative biologists, the negative finding for shoe size reported by Shah & Christopher is, in fact, unexpected. As a rule, all bodily structures of any species are adapted to fit overall body size, so in principle, the penis should tend to be longer in bigger men.
In fact, an early study by Loeb (1899) did report a fairly strong correlation between penis length and body height. And some more recent studies similarly reported significant findings. For example, a 2001 paper by Ponchietti and colleagues examining penis size and body size in 3,300 young men found highly significant correlations between flaccid penis length and body height and weight, although the latter relationship was negative.
However, it seems that disparities also arise because the penis is measured in different ways. It may be either flaccid or erect. To sidestep practical difficulties of measuring the erect penis, many investigators instead measure the length of the flaccid penis when stretched. Stretched and erect penis lengths are quite similar, so the former is often used as a proxy. But there is a fundamental problem because flaccid penis length is weakly correlated at best with the length of the stretched or erect penis. Many investigators have preferred to use stretched penis length as their basic indicator, so this may account for the puzzling lack of correlation with body size reported by some investigators.
It is notable, for example, that Ponchietti and colleagues found significant correlations when examining flaccid penis length, whereas Shah & Christopher measured stretched penis length in their study that found no correlation with shoe length. However, that cannot be the whole story as a smaller-scale study by Siminoski & Bain (1993) did report a significant correlation between stretched penis length and shoe size.
