Dennis McFadden received his doctoral degree in Sensory Psychology from Indiana University in 1967. That fall he began as an Assistant Professor at UT, and he has been here ever since. In 1984 he received a five-year Jacobs Javits Neuroscience Investigator Award from the NIH, in 1987 he was awarded a Piper Professorship, and in 1998 UT made him an Ashbel Smith Professor in Experimental Psychology. Dr. McFadden has been a member of the Executive Committee of the UT Institute for Neuroscience since 1985, and he recently became a member of the Executive Committee for the UT Center for Perceptual Systems. He has served as Associate Editor for Psychological Acoustics for the Journal of the Acoustical Society of America, and his research has been continuously funded by NIH since 1969. For many years, Dr. McFadden's research involved psychophysical measurements on various aspects of hearing--sound localization, masking, pitch perception, the aftereffects of exposure to intense sound, auditory adaptation, and comodulation masking, among other topics. In recent years, his interests have turned to more physiological measures of the auditory system, especially otoacoustic emissions (OAEs), which are sounds generated in the inner ear and monitored using a sensitive microphone system placed in the ear canal. Initially, Dr. McFadden was interested in the heritability of the individual differences observed in OAEs and that led to an interest in the sex differences seen in OAEs (females have more and stronger OAEs than males). While searching for possible explanations for the sex differences in OAEs, Dr. McFadden discovered that OAEs are atypically expressed in certain special populations of subjects. For example, monozygotic female twins have far more OAEs than all other females, females having male co-twins have far fewer OAEs than all other females, and female homosexuals and bisexuals also have far fewer OAEs than other females. The most plausible explanation for all of these facts is that prenatal exposure to androgens affects the expression of OAEs, an idea that is currently under test in two populations of rhesus monkeys that were exposed to androgenic or anti-androgenic agents during gestation. The OAEs of these monkeys are being measured and compared with the OAEs of control animals. In related work, Dr. McFadden has collaborated with Dr. Craig Champlin of UT's Department of Communication Sciences and Disorders to show that the auditory evoked potentials (AEPs) of homosexuals are also different from those of heterosexuals. Additional AEP studies are planned on other special populations of subjects. So, in this recent work on sex differences and special populations, Dr. McFadden has been using physiological measures of the auditory system as tools for studying phenomena related to prenatal development and sexual differentiation. Another recent interest is in the relationship between OAEs and the relative lengths of the index and ring fingers. This 2D:4D ratio also exhibits a large sex difference and large differences for certain special populations of subjects. The implication is that OAEs and 2D:4D may be under similar prenatal hormonal influences, and that implication is currently being tested experimentally.