Incest/Inbreeding Taboos

To support the deleterious theory of incest/inbreeding avoidance, human sociobiologists have repeatedly emphasized Edward Westermarck's hypothesis (1891) that children raised in close proximity will develop an aversion to sexual relationships with each other. Sociobiologists assume that this aversion originated as a naturally selected mechanism. Human sociobiologists site evidence from two case studies of human communities in support of Westermarck's hypothesis.

One group, the Israeli kibbutzim, separate children from their parents' household at birth and raise them in age-graded cohorts. In these cohorts boys and girls are raised without segregation, even sharing sleeping, bathing and toilet facilities; the proximity and intimacy of their upbringing is greater than what would usually be expected among siblings. Joseph Shepher (1983) studied these kibbutzim as a test of Westermark's hypothesis and reported that of the nearly 3,000 kibbutzim marriages he examined there was not one case of intra-cohort marriage.

However, several other researchers reported compelling research results which demonstrate that there are numerous social structural and ideological reasons why individuals of the same kibbutzim cohort might not marry (Talmon 1964; Spiro 1965). Mordecai Kaffman (1977), on the other hand, reported that by the late 1970s sex and marriage between cohort members had become common. John Hartung (1985), in re-analyzing Shepher's research, reported that not only did cohort members from Shepher's samples marry but did so at a disproportionately higher rate than would be expected for marriages involving non-cohort members.

A second case study, often sited in support of Westermark's hypothesis, was published by Arthur Wolf (1995). Wolf studied a form of Chinese marriage known as minor marriage (or sim pua marriage). In this marriage custom a family adopts a young girl and raises her as a sister to their son. This adopted sister will eventually be the son's wife. Because these marriages had a higher divorce rate and produced fewer children than Chinese "major" marriage, Wolf and the human sociobiology community presented this research as primary evidence in support of Westermarck's hypothesis.

Wolf makes it clear, however, that the Chinese consider sim pua unions to be low-status marriages for the poor, and these marriages are often the object of public ridicule and scorn. Adopted daughter-in-laws are frequently mistreated and unhappy, and given the suggestion of incest in a sibling relationship, it is a wonder that such marriages worked at all. In other words, there is no need to invent a complicated Darwinian mechanism to understand why the Chinese minor marriages more often failed.

Another area of research that refutes Westermarck's hypothesis is from studies of nudist communities. Dennis Craig Smith and William Sparks (1986) found that nudist children experience more sexual play with siblings and have more incestuous relations with immediate family members than non-nudists even though their early associations are quite intimate.