The risk of eating disorders in relatives of anorexic patients is higher than that found in control subjects (Table 6-6). Affective illness is also increased in relatives. Even if probands without a major affective illness are considered as a separate group, an excess of affective illness is observed in relatives compared to relatives of control subjects. In one study, 40 bulimic probands were compared to 24 control subjects. Relatives of bulimic probands had a 27.9% risk of major affective illness compared to 8.8% in relatives of control subjects. In comparison, the risk of major affective illness in relatives of bulimic probands without major affective illness was lower (19.1%), but this risk is still higher than for relatives of controls. The same study found that 11.8% of relatives of bulimic probands had an eating disorder, compared to 3.5% of relatives of controls. In the study of bulimia, relatives of probands also showed an excess of alcoholism and ASPD in comparison to relatives of controls.
One study reported pairwise concordance of 56% in MZ twins and 5% in DZ twins (71% and 10% with probandwise figures) (Table 6-7). Family history assessment (including additional informant data from parents) showed that 4.9% of female first-degree relatives and 1.16% of female second-degree relatives had had anorexia at some point in their lives, a risk considerably higher than the reported population prevalence. The MZ co-twins were much more similar in body dissatisfaction, drive to thinness, weight loss, length of amenorrhea, and minimum body mass index.
The heritability of the body mass index (weight in kilograms/height in meters squared) has been calculated as 0.77 at age 20 and 0.84 at age 45. A special sample of MZ twins raised apart (N = 93) was ascertained as part of the Swedish Adoption/Twin Study of Aging. The correlation for body mass index in that group was 0.70 for males and 0.66 for females. For MZ twins raised apart, the intrapair correlation coefficient may be considered a direct estimate of heritability.
Adoption studies of obesity in Denmark showed a significant correlation between body mass index of adoptees and that of their biological parents but not that of their adoptive parents. Genetic effects are less clear in the very obese, suggesting that environmental factors operate to determine the degree of obesity.
No linkage studies of eating disorders have been reported, but a large collaborative effort is in progress to accumulate data on sibling pairs with bulimia.