  Background: A wide range of diseases show some degree of clustering in
families; family history is therefore an important aspect for clinicians when
making risk predictions. Familial aggregation is often quantified in terms of a
familial relative risk (FRR), and although at first glance this measure may
seem simple and intuitive as an average risk prediction, its implications are
not straightforward.
  Methods: We use two statistical models for the distribution of disease risk
in a population: a dichotomous risk model that gives an intuitive understanding
of the implication of a given FRR, and a continuous risk model that facilitates
a more detailed computation of the inequalities in disease risk. Published
estimates of FRRs are used to produce Lorenz curves and Gini indices that
quantifies the inequalities in risk for a range of diseases.
  Results: We demonstrate that even a moderate familial association in disease
risk implies a very large difference in risk between individuals in the
population. We give examples of diseases for which this is likely to be true,
and we further demonstrate the relationship between the point estimates of FRRs
and the distribution of risk in the population.
  Conclusions: The variation in risk for several severe diseases may be larger
than the variation in income in many countries. The implications of familial
risk estimates should be recognized by epidemiologists and clinicians.
