This study examines the heritability of cigarette smoking, alcohol, and coffee consumption in 4,960 adult, male twin pairs (2,390 MZ and 2,570 DZ pairs) participants in an epidemiologic survey of the NAS-NRC Twin Registry conducted in the USA during 1972-73. Heritability estimates for smoking, alcohol and coffee use were calculated both before and after adjustment for shared variance between these behaviors and other demographic characteristics including socioeconomic status and an occupational adjustment score. The objective of the analysis was to determine the impact of adjustment for covariates on heritability estimates of smoking, alcohol and coffee use. Before adjustment, genetic effects in smoking, alcohol and coffee use accounted for 53%, 36%, and 45% of the variance, respectively. After adjustment, the corresponding estimates were 35%, 29%, and 36%. The fact that these estimates remained significant after adjustment for covariates leads to increased confidence about the role of genetic factors in substance use behaviors.
