When it comes to scientific studies of genitalia, whether it be human or otherwise, there's an unquestionable bias towards penises. As a new study shows, it's problem that's actually getting worse — and it's getting in the way of science.

No doubt, scientists just love their penises, including those of ducks, armadillos, turtles, echidnas, slugs, and fish. But when it comes to studying vaginas, not so much. By virtue of their new paper, "Genital Evolution: Why Are Females Still Understudied?," biologists Malin Ah-King and colleagues are hoping to raise awareness about this issue, arguing that an outdated single-sex bias is hampering our understanding of genital evolution. The paper's abstract explains it all very nicely:

The diversity, variability, and apparent rapid evolution of animal genitalia are a vivid focus of research in evolutionary biology, and studies exploring genitalia have dramatically increased over the past decade. These studies, however, exhibit a strong male bias, which has worsened since 2000, despite the fact that this bias has been explicitly pointed out in the past. Early critics argued that previous investigators too often considered only males and their genitalia, while overlooking female genitalia or physiology. Our analysis of the literature shows that overall this male bias has worsened with time. The degree of bias is not consistent between subdisciplines: studies of the lock-and-key hypothesis have been the most male focused, while studies of cryptic female choice usually consider both sexes. The degree of bias also differed across taxonomic groups, but did not associate with the ease of study of male and female genital characteristics. We argue that the persisting male bias in this field cannot solely be explained by anatomical sex differences influencing accessibility. Rather the bias reflects enduring assumptions about the dominant role of males in sex, and invariant female genitalia. New research highlights how rapidly female genital traits can evolve, and how complex coevolutionary dynamics between males and females can shape genital structures. We argue that understanding genital evolution is hampered by an outdated single-sex bias.

PLoS blogger Roli Roberts summarizes three obvious explanations:

a) Biological: Female genitalia don't vary enough to drive evolutionary change.

b) Practical: They do vary, and do drive evolution, but are devilishly hard to study.

c) Intellectual: They do vary and drive evolution, and can be studied, but the field is intellectually blinkered.

Read Roberts's entire summary of the paper here.

Image: Roman Sigaev/shutterstock.