158 Obesity
of nonidentical twins reared together, who have different genes but similar
environments. The results show that the identical twins reared apart are
more similar in weight than nonidentical twins reared together. For example,
Stunkard et al. (1990) examined the BMI in 93 pairs of identical twins reared
apart, 154 pairs of identical twins reared together, 218 pairs of nonidentical
twins reared apart, and 208 pairs of nonidentical twins reared together. The
authors concluded that genetic factors accounted for 66 –70 percent of the
variance in their body weight, suggesting a strong genetic component in
determining obesity, and stated that “genetic influences on body-mass index
are substantial, whereas the childhood environment has little or no influence.”
Similarly, Allison et al. (1996) estimated the heritability of obesity in twins
from the US, Finland, and Japan. They concluded that 50 percent of the
total variance found in BMI resulted from genetic factors. However, the
role of genetics appears to be greater in lighter twin pairs than in heavier
twin pairs.
Adoptee studies
Research has also examined the role of genetics in obesity using adoptees.
Such studies compare the adoptees’ weight with that of both their adoptive
parents and their biological parents. Stunkard, Stinnett, and Smoller (1986)
gathered information in Denmark about 540 adult adoptees, their adopted
parents, and their biological parents. The results showed a strong relationship
between the weight class of the adoptee (thin, median weight, overweight,
or obese) and their biological parents’ weight class, but no relationship with
their adoptive parents’ weight class. This suggests a major role for genetics
and was also found across the whole range of body weight. Interestingly,
the relationship to the biological mother’s weight was greater than the rela-
tionship to the biological father’s weight.
Research therefore suggests a strong role for genetics in predicting obesity.
Research also suggests that the primary distribution of this weight (upper
vs. lower body) is inherited (Bouchard et al., 1990). Furthermore it is argued
that the inheritance of obesity is controlled by both major gene effects and
polygenic effects. This means that for some individuals a single gene appears
to determine obesity, whereas for others obesity is related to a complex com-
bination of small effects resulting from a range of genes. A good review of
the role of genetics in obesity can be found in Kopelman (1999a). How
this genetic predisposition expresses itself is unclear. Metabolic rate, the
number of fat cells, and appetite regulation may be three factors influenced
by genetics.