This is a sociological study based on 96 well-documented cases of incest from the files of the Département de la Seine between 1937 and 1954. Cases investigated were limited to those where very complete material was available, and to father-daughter incest, which is the most prevalent. It is accepted that statistics on discovered incest are highly unreliable, and that for every case that appears in court, there are probably many that are never discovered. The analysis of case material is divided in two parts: 1. Incestuous families are studied as a whole, their social characteristics and those of each member of the family involved, in an attempt to trace etiological factors. Beyond this, a study is made of the social and personal characteristics which pertain to those actually guilty of incest, which may partly explain the phenomenon. 2. The second part consists of case studies which permit us to see the effect of incest on family relations, and its appearance in different types of family. This enables us to distinguish individual from collective factors in the appearance of incest and in its duration. A comparison is made with other sociological studies. Statistical information is both full and detailed. The majority of the fathers (58) were between 36 and 45 years of age. Most were married, or in a common-law relationship, and were mainly in the working or lower classes. Sixty-nine of the men had no previous criminal conviction, and the group was generally non-criminal, though 54 were chronic or occasional alcoholics. Thirty-nine were described as authoritarian or withdrawn. Statistics indicate a much higher than normal incidence of instability. Only nine could be considered normal. With regard to sexuality, it seemed within normal range; a large proportion were sexually deprived at the time of incest. The mothers in the sample were generally somewhat younger; 32 were working outside the home with resultant lack of supervision. From the point of view of personality, 48 were described either as dominating or submissive, 35 as devoted mothers, 14 as of doubtful morality. A larger than normal number were passive and submissive. The attitude of the mother towards the children is described as good in 40, indifferent or poor in 43. With regard to the daughters, 54 were regarded as normal, 42 as abnormal or psychopathic personalities. Mental development showed that 27 were intellectually normal, 31 dull normal, and 38 handicapped. Turning to the family as a whole, it was noted that they were somewhat larger than average. With regard to housing, while the majority were lower class, only 24 families could be described as living either in slum or overcrowded conditions. Economically, the situation in the family ranged from 12 in easy circumstances, 38 in good, 32 in mediocre and 14 precarious. The factor of poverty was therefore not decisive. Concerning marital relations, they could be described as good in only 19 cases, fair in 18, and bad to very bad in 49. Attitude of parents towards the children was 71 indifferent or poor, and 25 good. The majority of the daughters reported violence or threats on the part of the father, but this must be treated with caution. There is evidence of instances of genuine assent, impulsive or passive acquiescence. A number of brief case histories are presented describing different types of family structure and situation, precipitating factors, etc. An analysis of the cases shows that incest was not limited to one locale or social group or class, and that the effect within the family varied according to the value system, whether it was normal or marginal. In some families, incest could be accepted or assimilated, while in others the disclosure broke up the relationship, or the family, and was accompanied with guilt and remorse. The point is made that in some families early incest barriers were not well established, with consequent continuing attachment to sexual objects within the family. Socialization was incomplete to the point that what is generally considered abnormal was acceptable. In other situations, predisposing factors like the absence of the wife, provocative attitude of the daughter, alcoholism, depression of the father played an important part. The traumatic effects of incest on the daughter are discussed. Early sexual relations tend to affect both physical and mental balance; equally important is the restriction of normal social contacts. There is growing jealousy and possessiveness on the part of the father, which usually results in a disclosure of incest. The conclusion is that incest is not related to one factor, but to a congregation in which the social must be taken into account, as no human behaviour is ever completely intelligible from the light of individual psychopathology.