Bent pinkies: Quantifying fifth finger clinodactyly in a sample of U.S. adults.

Mild curvature of the fifth finger (or clinodactyly) is a relatively common trait. While severe forms can cause functional impairment and are a feature of certain congenital syndromes, mild clinodactyly is considered a minor morphological variant. Despite exhibiting continuous variation, clinodactyly is rarely treated as a quantitative trait. Consequently, the degree of fifth finger curvature in the general population and the factors that impact this curvature are not well understood. In the present study, we measured fifth finger curvature in a sample of 1,295 U.S. adults and investigated the role of sex, age and body size. We found that clinodactyly exhibited a non-normal distribution. All participants displayed some degree of curvature, but it tended to be slight with an overall mean of 3.68 degrees (median: 3.58 degrees). In only 0.8% of cases did the curvature exceed the nominal 10-degree threshold for clinically meaningful clinodactyly. We did not find statically significant sex differences. Further, there was no meaningful relationship with height and only a weak positive relationship with age. We found that clinodactyly showed asymmetry; the curvature was greater on the left than on the right fifth finger (p < 2.2e-16), but this was not influenced by sex, age, or height. These results suggest the possibility that the kind of ubiquitous mild clinodactyly observed in the general population may be etiologically distinct from more rare and severe forms of the condition.