Such congenital subnormal intensity or entire lack of the sexual impulse may be due to very various causes. According to _von Krafft-Ebing_, these causes may be organic or functional, mental or physical, and central or peripheral. The declining intensity of the sexual impulse with the advance of years, and the temporary disappearance of that impulse after the sexual act, are both physiological occurrences. Education and mode of life have a marked influence on the intensity of the vita sexualis. Strenuous mental activity, earnest study, severe physical exertion, mental depression, and sexual continence, notably diminish the excitability of the sexual impulse. At first, indeed, abstinence leads to an increase in the intensity of the impulse, but sooner or later the functional activity of the organs of generation declines, and therewith also the intensity of the sexual impulse. As peripheral causes of diminution or disappearance of the sexual impulse, _von Krafft-Ebing_ mentions oöphorectomy, degeneration of the reproductive glands, marasmus, sexual excess, whether in the form of coitus or of masturbation, and alcoholism. In like manner is to be interpreted the disappearance of the sexual impulse in general disorders of nutrition (diabetes, morphinism, etc.).
A decline in the intensity of the sexual impulse in consequence of degeneration of the conducting tracts, is found, according to _von Krafft-Ebing_, in diseases of the brain and the spinal cord. Central affection of the sexual impulse may be due to organic disease of the cerebral cortex (dementia paralytica, general paralysis of the insane, in the later stages), or it may be due to functional disorder, such as hysteria, or to mental diseases (melancholia or hypochondriasis).
Finally, in some instances, the sexual impulse in women manifests itself, not in the normal manner with copulation with the male as its goal, but in a form demanding some abnormal kind of gratification (psychopathia sexualis), whether it be because sexual intercourse with the male affords the woman no enjoyment, or simply because no opportunity exists for such intercourse.
Masturbation is very frequent; the habit having been acquired from bad example by the girl during the menarche, it is sometimes continued by the wife during married life. In these cases we often find distinct changes in the genital organs, such as hypertrophy of the clitoris, enlargement and bluish discoloration of the nymphæ, retroversion of the uterus, tenderness and displacement of the ovaries, considerable vaginal discharge, and sometimes menorrhagia.