The simultaneous assessment of constitutional, behavioral, and environmental factors in the development of large nevi.

BACKGROUND The presence of large nevi (>4 mm diameter) is the most important predictor of melanoma risk. We report on the simultaneous assessment of behavioral, constitutional, and environmental risk factors for the prevalence of large nevi in a population-based study of 33,305 individuals and compare those to risk factors for melanoma. METHODS We used self-reported data from a population-based study of twins in California, assessing the prevalence of large nevi, lifetime usual sun exposure behavior and sunburn experience, skin type and color, and birthplace and recent domicile, for which we obtained estimates of potential sun exposure. RESULTS Among constitutional variables, skin type (burn rather than tan) and having blond (but not red) hair color were associated with having more than three large nevi, as was Celtic ancestry. Spending more time in the sun in adulthood was inversely associated with number of large nevi, whereas spending time in the sun during weekends in childhood increased the risk of having large nevi. We observed no latitude gradient in nevi prevalence, except among those of Mediterranean ancestry (those in the South were more likely than those in the North to report large nevi). CONCLUSION The prevalence of large nevi was most readily explained by constitutional and then by behavioral variables in this study, rather than by independent variables describing an individual's opportunity for sun exposure. There seem to be independent relationships between nevi frequency and other melanoma risk factors, with the notable exceptions of skin color and red hair phenotype: implying that if these risk factors represent a genetic propensity to develop melanoma, that risk would seem to be unrelated to the development of nevi.