These studies were designed to assess the influence of sun exposure on nevi in white people. To eliminate the confounding effect of age, sex, and phenotype, two parallel studies were conducted on people of the same age (17 to 24 years; median, 20 years old), sex (male), and phenotype: one in people with "red" phenotype (red or red-blond hair, white complexion on the inner part of the arm, and inability to tan) and one in people with "dark" phenotype (brown or black hair, dark complexion on the inner part of the arm, absence of freckles, and easy tanning without burning).
In both groups, comparison of nevus counts on the inner and outer side of the upper extremities and comparison of mean density of nevi (number per square meter) in always-exposed and never-exposed skin show that the number of nevi is higher in sun-exposed areas. The density of large and atypical nevi was maximal on intermittently sun-exposed skin while the density of small nevi was maximal on always-exposed skin. The number of large nevi on intermittently exposed skin correlated with cumulative intensive exposure during beach recreation in the red phenotype group. The number of large nevi was significantly higher in red phenotypes who repeatedly experienced severe sunburns in their first 20 years of life.
The number of nevi at the end of the second decade is influenced by cumulative sun exposure from birth. "Traumatizing" sun exposure, which is more frequent in the red phenotype than in the dark phenotype, has an influence on the number of large nevi and is therefore likely to make small nevi grow.(Arch Dermatol. 1993;129:1280-1285)
Richard MA, Grob J, Gouvernet J, Culat J, Normand P, Zarour H, Bonerandi J. Role of Sun Exposure on NevusFirst Study in Age-Sex Phenotype-Controlled Populations. Arch Dermatol. 1993;129(10):1280-1285. doi:10.1001/archderm.1993.01680310050007