Do we know each other?
Did King Oedipus have a chance? After all, he couldn't smell that this beautiful older woman was his mother. Or is it? Relatives may also recognize each other by smell - but it works quite differently between brother and sister.
Some rules of man are older than his language and have survived from the first Stone Age thinker to the present day. These rules can then be found, in different forms but with almost the same meaning, in almost all peoples of the world, despite the most varied community structure. One of these eternal laws of mankind is called "blood is thicker than water" in European.
Blood means kinship here - brothers, sisters, mothers, fathers and the rest of the clan, that's what the Middle High German vernacular said in the 12th century. Century, when in doubt, stick to each other more than to mere friends or even clear strangers. The vernacular is often right: When it comes to money, help and favours, relatives are really given preference in wills or emergencies. But when it comes to love and sex, an eternal rule becomes an equally ancient taboo – having sex with kindred blood is “blood shame”.
Debra Lieberman of the University of California at Santa Barbara and her colleagues are now concerned with what lies behind this total altruistic support of relatives on the one hand and the incest taboo on the other. She was less interested in the natural meaning behind the protection within the family and the social ban on family sex contacts, which in both cases is already well founded scientifically. Rather, the researchers asked themselves how people have been able to identify siblings with certainty, from the early Stone Age hunter communities to the present day.
Of course it could have been said at some point by mom or dad or, in pre-linguistic early human clans, it could have been shown through deeds – this is your brother, your sister. But perhaps this is not necessary at all, as sociological observations in the century before last had already shown. In 1891, the Finn Evard Westermarck had put forward a hypothesis about natural avoidance of incest, which over time seemed to turn out to be more and more true: simply spending a long time together in childhood ensures that a counterpart is no longer classified as sexually attractive later on - or, the other side of the coin, helping him altruistically more than others.
However, it does not matter whether there is actually a relationship with the fellow-child: marriages concluded between people who had grown up together, for example as wards or step-siblings, were, according to the first anthropological-statistical evaluations of the 19th century. In fact, at the end of the 20th century, people were less happy, had fewer children, were less passionate, and were more likely to get divorced. Over time, the evidence for this theory became less and less anecdotal and more scientifically sound. Even today, however, one of the most common hypotheses on the biological background of the human incest taboo is that children who are brought up together in a certain formative phase usually later develop no sexual interest in each other.
Lieberman and colleagues have now extended this theory and tapped it for possible weak points with the help of a computer model and the data from a survey of 600 volunteers. Participants had either younger or older siblings with whom they had spent their entire childhood or with whom they had had little or no upbringing. For example, the researchers asked everyone about their level of reluctance to engage in sexual activity themselves with a sibling and their moral attitude towards incest in third parties. Respondents were also asked to indicate how selflessly they would help siblings or how sympathetic they would be if others did so at the expense of others.
As expected, people with siblings showed clear aversions to ideas of incest and were also markedly more altruistic - although not always to the same extent. And interestingly, the time that siblings spent together in childhood was not always a decisive criterion for the extent of typical sibling behavior. In the case of people who had younger siblings, it didn't really matter whether they were raised with them for many years or only for a very short time. Not so for candidates with older siblings: they were very altruistic and strongly opposed to inbreeding, especially if they had spent a long time with their brothers. However, the longer they were raised alone, the less pronounced they showed the typical defenses against incest.
Lieberman's team had expected this result even before the survey. In her opinion, the duration of a common childhood is only the second best strategy that people use to recognize their siblings unconsciously. It is much more formative when a child perceives that another child remains in close contact with its own mother for a certain period of time shortly after birth, or, more scientifically, when it sees with its own eyes a "maternal perinatal association (MPA)" of the child observed.
Of course, this MPA can only observe an older sibling in a child born later - if this is the case, then the mothered infant is classified as a sibling, and as a result the later attitude towards it, which becomes clear in the questionnaire, also changes altruism within the family and the strong rejection of incest - regardless of whether the latecomer was raised together in the family for a long time. But it is different with younger children, who were not even born during a phase of close postnatal association between mother and older brother: They must be missing the observation of the MPA, which is why they have to resort to a different form of sibling recognition. This is the duration of their childhood together. If it is only short, the rejection of incest and the understanding of altruism decrease significantly, as confirmed by the evaluation of the questionnaires.
Also their conclusions are based only on correlations and should be interpreted with caution, Lieberman and colleagues point out. However, there is much more to be said for their model than for alternative explanations. For example, it cannot be conclusively deduced from their data that only an intact family living together permanently, in which stable moral or religious values are possibly better conveyed, later leads to a stronger rejection of incest. On the other hand, the fact that among male respondents with such family constellations a strong general rejection of inbreeding can only be observed if they had a sister, but not if they had a brother. The later attitude can therefore not be fed solely from family moral considerations, but clearly has a strong biological component.
It cannot be ruled out that other signal stimuli signal kinship, according to the researchers - an odor component could play a role and the main histocompatibility complex, for example, reveal kinship - a very individual protein that is omnipresent in the cells and that characterizes every human being, probably plays a role in choosing a partner and is more similar in relatives than in strangers. Other scientists speculate that external similarities may somehow be accounted for neurologically. However, such factors can hardly play a major role: Even in the case of people who are in fact not genetically related, but who only found out about it as adults, the duration of their common childhood played the all-important role in Lieberman's test. For firstborns, the MPA, i.e. the formative observation of an intimate relationship between their own mother and an unknown infant, is the best insurance that has been proven to date for brotherly – and against sexual feelings.
