Glenn Wilson, PhD, Associate Professor at the Institute of Psychiatry at King's College in London and Qazi Rahman, PhD, Lecturer in Psychobiology at the University of East London, stated in their 2005 book Born Gay: The Psychobiology of Sex Orientation:
"Scientists and lay people alike have used several ways of classifying people into gay, straight, lesbian, or somewhere in between... Among the classification criteria used are:
the labels people apply to themselves (e.g. 'gay', 'lesbian', 'bisexual' or 'straight')
their actual sexual behavior (e.g. numbers of male versus female partners in the past and present)
self-reported sexual feelings (e.g. fantasies and desires)
genital or brain responses (physiologically measure arousal to male and female images)
These different methods for measuring sexual preferences do not always agree, which raises the question as to which are the best indicators... [P]hysiological measures may be the best means of all of assessing sexual orientation.
By observing genital responses (e.g. a penis volume measure for men or vaginal blood flow for women) to erotic stimuli (pictures of nude men or women), or brain responses (by EEG or functional MRI scans), we obtain objective information regarding an individual's erotic preferences and arousal patterns.
When self-reported desires and physiological responses agree, there is no problem; when they provide conflicting information, the latter are perhaps more telling because they are less susceptible to social pressures or conscious impression management."
Nathaniel McConaghy, MD, former Associate Professor of Psychiatry at the University of New South Wales, stated in his 1999 article "Unresolved Issues in Scientific Sexology," in Archives of Sexual Behavior:
"Evidence of the validity of penile volume assessment of the sexual orientation of individuals was provided by Freund (1963). All of 65 heterosexual men and 48 of 58 homosexual men were classified correctly. The finding was replicated. All of 11 men who identified as heterosexual were correctly classified and 17 of 22 men seeking treatment for compulsive homosexual feelings were classified as predominantly homosexual; prior to assessment, 3 of the 5 classified as predominantly heterosexual had reported that they were (McConaghy, 1967)."