In a recent article, Lewis et al established a risk estimation model for individuals whose siblings were diagnosed with Crohn disease by incorporating data on CARD15 (NOD2) genotypes, smoking and family history.1 They used Crohn disease as a well selected model for a complex disease,2 which is a timely example in an era when new genetic data on human diseases is filling all media and when discussions abound about individualised genomic medicine.3 4 We would like to consider three separate issues emanating from risk estimation for complex diseases at an individual and population level: reliability of familial risk estimates, use of the low genotype relative risks (GRR) and implications of genomic data at the population level (relating to population attributable fraction, PAF, and familial risk).

An overwhelming proportion of the global literature on familial risks is based on anecdotal information on disease in relatives. There is ample literature illustrating the problem of false reporting of cancers in family members; for some reported cancers less than a half can be confirmed by medical records.5 6 The problem is even worse because the direction of false reporting is often opposite for the cases and the …