Reporting bias in the association between age at first alcohol use and heavy episodic drinking.
BACKGROUND
Given the weight placed on retrospective reports of age at first drink in studies of later drinking-related outcomes, it is critical that its reliability be established and possible sources of systematic bias be identified. The overall aim of the current study is to explore the possibility that the estimated magnitude of association between early age at first drink and problem alcohol use may be inflated in studies using retrospectively reported age at alcohol use onset.


METHODS
The sample was comprised of 1,716 participants in the Missouri Adolescent Female Twin Study who reported an age at first drink in at least 2 waves of data collection (an average of 4 years apart). Difference in reported age at first drink at Time 2 versus Time 1 was categorized as 2 or more years younger, within 1 year (consistent), or 2 or more years older. The strength of the association between age at first drink and peak frequency of heavy episodic drinking (HED) at Time 1 was compared with that at Time 2. The association between reporting pattern and peak frequency of HED was also examined.


RESULTS
A strong association between age at first drink and HED was found for both reports, but it was significantly greater at Time 2. Just over one-third of participants had a 2 year or greater difference in reported age at first drink. The majority of inconsistent reporters gave an older age at Time 2 and individuals with this pattern of reporting engaged in HED less frequently than consistent reporters.


CONCLUSIONS
The low rate of HED in individuals reporting an older age at first drink at Time 2 suggests that the upward shift in reported age at first drink among early initiates is most pronounced for light drinkers. Heavy drinkers may therefore be overrepresented among early onset users in retrospective studies, leading to inflated estimates of the association between early age at initiation and alcohol misuse.