Otoacoustic emissions (OAEs) differ between the sexes in humans, rhesus and marmoset monkeys, and sheep. OAEs also are different in a number of special populations of humans. Those basic findings are reviewed and discussed in the context of possible prenatal androgen effects on the auditory system. A parsimonious explanation for several outcomes is that prenatal exposure to high levels of androgens can weaken the cochlear amplifiers and thereby weaken otoacoustic emissions (OAEs). Prenatal androgen exposure apparently also can alter auditory evoked potentials (AEPs). Some non-hormonal factors possibly capable of producing sex and group differences are discussed, and some speculations are offered about specific cochlear structures that might differ between the two sexes.
KeywordsOtoacoustic emissions; Auditory evoked potentials; Sex differences; Prenatal development; Masculinization; Testosterone; Estradiol Address for Corresponding Author and manuscript correspondence: Dennis McFadden, Department of Psychology, Seay Building, University of Texas, 1 University Station A8000, Austin, TX 78712-0187, (512) 471-4324, (512) 471-5935 (fax), E-mail: mcfadden@psy.utexas.edu. 1 In science, the words sex and gender have distinctly separate meanings and should not be used as synonyms (e.g., Wizemann and Pardue, 2001, p. 176). Sex alludes to a person's body morphology, and gender alludes to a person's self-image as belonging to one (or neither) of the two sexes. Assessing gender requires substantially different procedures from assessing morphological sex. Many investigators who have used the term gender actually did not measure it. 2 Pairwise comparisons like sex differences are commonly characterized using effect size, a d′-like measure that expresses the difference between means in standard-deviation units. We calculate effect size as the difference between the means of the two groups of interest divided by the square root of the weighted mean of the two variances. By convention, effect sizes of 0.2, 0.5, and 0.8 are viewed as small, medium and large differences, respectively (Cohen, 1992). I have provided effect sizes in the figure captions. 3 One possible reason for this inconsistency in the sex differences for DPOAEs and CEOAEs is the effective level of the stimuli used to elicit the two forms of OAE. DPOAEs typically are measured with primary tones of 50 dB SPL or greater, whereas CEOAEs typically are measured with clicks of 60 -80 dB peSPL (peak-equivalent sound-pressure level). This means that the spectrum level of the clicks is quite weak at each of the successive locations along the length of the cochlea contributing to the reflection-based component of the CEOAE, while the energy in the primary tones used to evoke DPOAEs is relatively more concentrated. Accordingly, sex differences typically are measured using locally weak stimuli for CEOAEs and locally strong stimuli for DPOAEs, and this difference eventually may prove relevant. 4 Sometimes it is worthwhile to say the obvious. Having a masculinized coch...