Aims To incorporate a psychosocial model of tobacco smoking into a behaviour genetic design to examine genetic and environmental influences on variation in smoking involvement.
Design Longitudinal twin study.
Setting and participants Twins initially aged between 13 and 18 years and registered with the Australian Twin Registry were surveyed three times between 1988 and 1996. A total of 414 pairs of identical and same-sex fraternal twin pairs participated in all three surveys, aged between 20 and 25 at wave 3. Biometric modelling estimated the influence of genetic and environmental factors in determining variation in smoking at each wave, both before and after adjusting for perceived smoking behaviours of peers and parents.
Measurements Twins answered a questionnaire on their own smoking status and reported on the use of tobacco by parents and friends as they perceived it, at each survey wave.
Findings At all three surveys, current smokers were more likely to have parents who smoked and to have smokers among their peers. Genes and environmental factors, both common and unique, contributed to variation in smoking behaviours. However, after controlling for the smoking behaviours of peers and parents, the role of genes in determining variation in smoking involvement was reduced by 100% at wave 1 and by 30% at wave 2. Friends’ smoking reduced the magnitude of the common environment variance by 11%, 30% and 40% at waves 1, 2 and 3, respectively. Parents’ smoking behaviours explained part of the common environment. Biometric modelling of the covariation between smoking involvement and peer smoking suggested that genes might influence smoking involvement at wave 1 by influencing choice of peers.
Conclusion Environmental factors play the greatest role in determining variation in tobacco smoking among adolescents and young adults. Among adolescents, genes may influence variation in smoking behaviours indirectly by influencing choice of peers. However, genes seem to have a direct influence on variations in the smoking behaviours of young adults.