Spontaneous otoacoustic emissions (SOAEs) were measured in human monozygotic (MZ) and dizygotic (DZ) twins and in a sample of non-twins. The number of SOAEs exhibited was more highly correlated in MZ co-twins than in same-sex DZ co-twins. Model-fitting to the correlations suggested that about three-quarters of the individual variation in the expression of SOAEs is attributable to genes. There was no convincing evidence for the heritability of specific SOAE frequencies. In accord with past surveys, SOAEs were more numerous in right than left ears, and in female than male subjects. Also investigated were the numbers of SOAEs exhibited by dark-versus light-eyed people and by MZ versus DZ twins. Those differences in our data were small and not statistically significant, but they were in a direction consistent with other studies: more SOAEs in dark-eyed individuals and in MZ twins. The view presented here is that SOAEs themselves are unlikely objects for natural selection, and probably are epiphenomena resulting from selection for those cochlear mechanisms that contribute to good hearing sensitivity - which is related to SOAE expression. It is argued that, in addition to genetics, other factors have the potential to affect the specific numbers of SOAEs that are expressed. For example, some aspects of the complex prenatal process of producing a male fetus are presumed to be responsible for the smaller number of SOAEs seen in males than females.