Short CommunicationHeritability of cannabis initiation in Dutch adult twins
Introduction
Cannabis is a commonly used drug worldwide. An estimated 166 million people used cannabis in 2006/7, equivalent to about 4 percent of the global population aged 15-64 (World Drug Report 2008).
Cannabis use is associated with increased risk for the subsequent use of more harmful drugs such as cocaine and heroin (Lynskey, Vink, & Boomsma, 2006) and higher risk to psychotic symptoms (Chen & Lin, 2009). Therefore it is important to know what causes people to initiate cannabis use. Twin studies can be used to disentangle the magnitude of genetic and environmental influences. Previous studies have reported both genetic and environmental factors as significant contributors to cannabis initiation. The heritability estimates ranged from 13% to 72%, while the shared environmental influences ranged from 0% to 68% (Kendler et al., 2000, Lynskey et al., 2002, Maes et al., 1999, McGue et al., 2000, Miles et al., 2001, Rhee et al., 2003, Shelton et al., 2007). These studies were mainly done in the United States, Australia and United Kingdom. In contrast to those countries, the use of cannabis in small amounts is, although not legal, permitted in the Netherlands. In the present study we use data from a sample of Dutch twins to examine the heritability of cannabis initiation in a country with a liberal cannabis policy.
Section snippets
Sample
Subjects are registered with the Netherlands Twin Register (NTR) (Boomsma et al., 2006). Most of them participate in longitudinal studies of health, lifestyle and personality. For this study we focused on the data from the 2000 survey (Vink and Boomsma, 2008, Vink et al., 2007), which was completed by 4609 twins. We selected participants between 21 and 40 years (N = 3115). Mean age of the subjects was 27.4 years old (SD = 4.7). Zygosity was based on DNA tests or on questions concerning similarity.
Cannabis initiation
The
Results
The prevalence in cannabis initiation did not differ for MZ or DZ twins (Table 1, model 1b), indicating that there is no process of social interaction between the co-twins. Constraining the thresholds to be the same in men and women resulted in a significant worsening of the model fit (Table 1, model 1c). The prevalence of cannabis initiation in men was 36.2% compared to 24.7% in women. The tetrachoric correlations derived from the best fitting model (model 1b), which allowed for different
Discussion
The prevalence of cannabis initiation was significantly higher in men than in women, which is in line with other studies (Degenhardt et al., 2008). The heritability estimates for cannabis initiation were the same in both sexes. The size of the estimate (44%) seems in line with other studies exploring the heritability of cannabis initiation. However, it should be noted that the heritability estimates in other studies ranged from 13% to 72%. This could be due to several factors, like phenotypic
Web resources
World Drug Report: http://www.unodc.org/unodc/en/data-and-analysis/WDR-2008.html
Netherlands Twin Register: www.tweelingenregister.org
Role of funding sources
This study was funded by NWO grant 985-10-002, by ZonMW (Addiction) grant 31160008 and by NIH DA018673 (Psychometric and genetic assessments of substance use, PI Neale). Dr. Vink is financially supported by NWO (VENI 451-06-004). These organizations had no further role in study design, in the collection, analysis and interpretation of data, in the writing of the report, or in the decision to submit the paper for publication.
Contributors
L.W. carried out preliminary analysis and a literature search. J.V. undertook the final analyses and wrote the manuscript. D.B. and M.N. helped in the design and implementation of the study and in the interpretation of the results. All authors contributed to and have approved the final manuscript.
Conflict of interest
All authors declare that they have no conflicts of interest.
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