Why we conform to the last taboo
By Patrick Bateson and Paul Martin National Post
22 September 2000
While there are strong biological reasons for avoiding
sexual relations with close kin, social conditioning has
always been a key reason, too.
At the back of the Anglican Book of Common Prayer is
a table of Kindred and Affinity "wherein whosoever are
related are forbidden by the Church of England to marry
together." The table stipulates that a man may not marry
his mother, his sister, his daughter, or numerous other
relatives, and neither may a woman marry her uncle,
nephew, grandparent or grandchild.
These elaborate rules are typical of incest taboos found
in most cultures, although taboos usually relate to
forbidden sexual relations rather than marriages. At first
sight the rules look as though they make good biological
sense, because they seem to minimise the genetic risks
But the list in the Book of Common Prayer continues
with some surprising prohibitions. For example, a man
may not marry his wife's father's mother or his
daughter's son's wife. The mind boggles at the thought of
the need for such rules. What was life like in the 16th
century if the church felt it necessary to specify such
prohibitions? And how many women had the
opportunity even to contemplate marrying their
deceased granddaughter's husband?
At least six of the 25 relationships that are expressly
prohibited from developing into marriage involve no
genetic link at all. And yet the Church of England did not
prohibit marriages between first cousins. Some other
cultures do, but here again the inconsistencies are
striking. In some cultures marriages between "parallel"
first cousins are forbidden, whereas marriages between
"cross" cousins are allowed and even actively
encouraged. (A parallel cousin is the child of the father's
brother or the mother's sister, whereas a cross cousin is
the child of the father's sister or the mother's brother.)
To biologists with an over-simplified view of the
evolutionary origins and biological functions of incest
taboos, this distinction is puzzling, because in biological
terms parallel and cross cousins do not differ. The
genetic relatedness is the same in both cases.
An incest taboo is a culturally transmitted prohibition.
This is not the same as a natural sexual preference for
non-relatives. "Incest", like "rape" and "marriage", is a
term that is sometimes mistakenly applied to animals in
an attempt, perhaps, to lighten normally dull scientific
But the taboos are embedded in human institutions with
all their associated restrictions, rights and
responsibilities. There is no reason to suppose that
anything comparable can be found in other animals.
However, humans are like many other species in that we
are normally disinclined to have sexual relationships with
individuals with whom we have grown up from an early
age. Such preference for novelty occurs even when
children are encouraged by their family to marry a
person who is familiar from early life.
Arthur Wolf, an anthropologist at Stanford University,
has spent a lifetime studying the arranged marriage
systems that used to be found in Taiwan. In some of the
marriages, the wife-to-be was adopted into the family of
the husband-to-be when she was a small child. These
marriages were conspicuously unsuccessful when they
were eventually formalised and were much more likely
to end in divorce – which itself was deeply frowned
upon. The partners simply did not find each other
sexually interesting and broke up in the face of strong
Interestingly, however, the marriages were only
unsuccessful if the girl had been adopted into the family
of her future husband when she was aged five or less. If
she was older, they did not differ from marriages in
which the partners met when they were mature.
The propensity to avoid mating with very familiar
members of the opposite sex may be seen as a product
of biological evolution. In the past, those individuals who
learned about the people with whom they were
associated in early life and chose mates who were a bit
different would be more likely to have healthy children
than those who didn't. This would have happened
because they would have been less likely to inbreed with
Superficially, the cultural prohibitions that have been laid
down in the incest taboos of churches and states look as
though they have been designed to prevent inbreeding
even though their evolution and their function are quite
different from the unconscious sexual preferences arising
from early experience. In our book Design For A Life,
we describe one possible solution to the surface
similarity between inbreeding avoidance and incest
Humans typically disapprove of other people behaving
in ways in which they would not themselves behave.
Left-handers were once forced to adopt the habits of
right-handers because the majority right-handers found
the minority left-handers' behaviour disturbing. In the
same way, individuals who had sex with close kin –
something few people would wish to do – were
Most people who had grown up with close kin were not
sexually attracted to them, and disapproved when they
encountered others who were attracted to theirs. This
disapproval had nothing to do with society seeking to
avoid being saddled with the half-witted products of
inbreeding, because few societies had any idea that
inbreeding was the cause. In any event, even if they did
know about the ill-effects of inbreeding, having healthy
offspring oneself is not the same as encouraging others
to have healthy offspring.
We believe that the disapproval was about suppressing
abnormal behaviour that could disrupt small, closely-knit
societies. Such conformity seems harsh from a current
perspective. But in the hunter-gatherer environment in
which humans evolved, unity would have been crucial
for survival. Wayward behaviour could have destructive
consequences for everybody. It is easy to see why
social conformity became a powerful trait.
Once in place, the desire for conformity on the one
hand, and the unconscious inhibitions against inbreeding
on the other, would have combined to generate social
disapproval of inbreeding. Much later in human history,
this disapproval was codified in elaborate social rules
According to this theory, the particular incest taboos of
a society should depend on which sorts of people were
likely in practice to grow up together.
This explains at least some of the apparently strange
proscriptions in the Church of England's marriage rules.
Early familiarity also explains why some societies
prohibit parallel cousins from marrying but favour
marriages between cross cousins. In these cultures,
parallel cousins tend to grow up together because in
practice brothers tend to live near brothers and sisters
near sisters. In contrast, cross cousins tend to be less
familiar with each other because brothers and sisters
usually live in different places after marriage.
If these ideas are correct, then human incest taboos did
not arise historically from a conscious intention to steer
others away from the biological risks of inbreeding.
Rather, two quite separate mechanisms were at work
during human evolution. One was a developmental
process designed to strike an optimum balance between
inbreeding and outbreeding when choosing an ideal
mate. The other was social conformity.
When these two propensities were put together during
the evolutionary history of our species, the product was
social disapproval of individuals who chose mates from
within their close family. Later in human history, more
formal rules appeared which could be transmitted from
generation to generation – initially by word of mouth and
subsequently in written form. And hence a man may not
marry his wife's father's mother or his daughter's son's
The general message from this story is that when we
seek to understand ourselves, biologists are not going to
provide all the answers any more than the social
scientists will. The biological processes that underlie
each individual's development often have a regularity,
but they can generate lives that are quite unique.
Consider the rules that are required for a game of chess.
They are clear-cut and nobody is allowed to change
them. And then think how complex are the games that
arise from such simplicity. The strategies involved
provide endless interest and nobody would suppose that
you would be able to grind them out from a simple
knowledge of the rules.
Incest taboos provide another example from human
social behaviour where, to change the metaphor, if you
want to admire a cathedral don't look at the shape of the
Professor Patrick Bateson is Provost of King's College. He and Dr Paul Martin are authors of 'Design For A Life'. The paperback is published by Vintage (£7.99) Up
© 2000 Independent Digital (UK) Ltd.
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