FINNISH SOCOLOGIST Edward Westermarck proposed the first leading theory about sibling incest. Westermarck (1862-1939) suggested siblings bond and form a nonsexual relationship with one another during the "critical period" - the first six years of life. If siblings are separated during that time for a year or more, the chances for later sexual contact appear to increase. Until recently no one had really tested out Westermarck's theory directly. So York psychology professor Irwin Silverman and doctoral student Irene Bevc decided to investigate. A 1993 study of sibling incest in context of the nuclear family was the first of its kind. Since then, they've done a follow-up study to refine their findings. They interviewed 43 people who had full intercourse with their siblings, 35 who had post-pubertal sexual relations of some kind with their siblings (but not genital intercourse), and 80 people who had never had any sexual activity with their brother or sister. Subjects were asked questions about nudity in the home, how much intimate time they had spent with their brothers or sisters, and the time and extent of any separations of more than a year in early childhood. They found 16 out of the 43 respondents who had genital intercourse with a sibling had been separated from that sibling for more than one year of the first six. Of these 16, 12 had been apart for more than four years.

Subjects were asked questions about nudity in the home, how much intimate time they had spent with their brothers or sisters, and the time and extent of any separations of more than a year in early childhood. They found 16 out of the 43 respondents who had genital intercourse with a sibling had been separated from that sibling for more than one year of the first six. Of these 16, 12 had been apart for more than four years. In both studies there was a greater incidence of early separation in the group reporting genital intercourse, there were no differences in frequency of separation in the group reporting less mature forms of sexual activity. Siblings reporting less mature forms of sexual behaviour tended to report more physical contact and mutual nudity during childhood. There was also a greater incidence of genital intercourse among non-biologically related sibling pairs, regardless of early separation or intimacy. Bevc and Silverman say their findings "broadly support" the Westermarck hypothesis. Growing up together may not remove all forms of sexual interest between siblings, they say. But it does appears to create a clear distinction between vague sexual interest and full genital intercourse. They are now planning a new study to examine how early separation is related to increased incidence of father-daughter sexual activity.