Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm

Abstract

The evolutionary significance of the female orgasm is unclear. Adaptationist accounts seek to ascribe a positive fitness benefit to females that achieve orgasm during vaginal intercourse, spawning theories that propose the female orgasm is an adaptation to bipedalism, communal life, pair-bond formation, insemination efficiency, and mate quality assessment, among others (reviewed in Lloyd 2005). Structuralist accounts explain female orgasm as a byproduct of the evolutionary, developmental, and physiological processes that generate the male orgasm (Lloyd 2005). Thus, the byproduct hypothesis asserts that the female orgasm results from shared evolutionary and developmental processes with males and is not the result of direct selection on the trait in females. The primary evidence cited in favor of the byproduct hypothesis is the high variability in womens’ ability to achieve orgasm during vaginal intercourse compared with the near certainty of male orgasm (Lloyd 2005). A prediction of this hypothesis is that a lack of selection on female orgasm releases the genital structures responsible for triggering orgasm in women (the clitoris) from selection, leading to greater phenotypic variation than for structures under stronger selection pressure for achieving an orgasm (i.e., the penis) (Wallen and Lloyd 2008). Wallen and Lloyd (2008) recently used this expectation to examine the variability in clitoral and penile lengths for evidence of differential selection pressure. Comparing the coefficient of variation in length between the clitoris and penis from large datasets (Table 1), Wallen and Lloyd (2008) found that clitoral length was significantly more variable than penile length (P50.002). These authors concluded, ‘‘given the close connection between the clitoral structures and womens’ orgasms, this marked variability in clitoral [length, compared with variability in penile length,] suggests little or no selective pressure on clitoral structures’’ (Wallen and Lloyd 2008). However, this conclusion rests on two important assumptions: (1) clitoral length is directly related to a womens’ ability to achieve orgasm, and (2) length is the best metric for the size of the penis and clitoris. Available data refute the former assumption, while the latter is clearly a one-dimensional view of size that may not be an accurate measure of ‘‘size.’’ A more realistic metric for size should include a description of both length and width, such as volume. Therefore, I performed the same analysis as Wallen and Lloyd (2008), but used volume measures instead of length. Unlike the results reported by Wallen and Lloyd (2008) for length data, clitoral volume was not significantly different from penile volume (1.39-fold difference, P50.17; Table 1). Following the rationale of Wallen and Lloyd (2008), volume data suggest that the size of the clitoris and penis are under similar selection pressures, in contrast to their conclusion for length data. This result is particularly surprising given the strong signature of co-evolution between penile and vaginal length reported by Wallen and Lloyd (2008), which suggests that clitoral size (as measured by volume) is under particularly strong stabilizing selection despite variability in length among women. More generally, the primary argument used to support the byproduct hypothesis for the origin of the female orgasm is its uncertain occurrence during sex, which is taken as evidence for reduced or absent stabilizing selection on female orgasm. Thus, Wallen and Lloyd (2008) assume that the clitoris, as the structure primarily responsible for generating the orgasm (Narjani 1924; Masters and Johnson 1966), must also be free from stabilizing selection and this will be reflected in variation in its size. This relationship is essential for their conclusions, however, there is no relationship between clitoral size and ability to achieve orgasm (Masters and Johnson 1966). For that matter, it seems unlikely that penis size is directly related to a male’s ability to reach orgasm during sex. Indeed, the ape penis is much smaller than the human penis with the average chimpanzee’s penis approximately 14-cm long, and the average gorilla and orangutan penis only approximately 3-cm long (Diamond 1992). Yet I doubt proponents of the byproduct hypothesis would conclude that males of these species experience orgasm during intercourse less frequently than human males. Attempts to explain the female orgasm have provoked a growing and sometimes heated debate (Barash 2005; Judson 2005; Zuk 2006; see Amundson 2007 for a thoughtful review). Yet, empirical data to test hypotheses, both functional and EVOLUTION & DEVELOPMENT 10:4, 396 –397 (2008)