Monozygotic twins have a higher concordance of alcohol dependence than dizygotic twins and since monozygotic twins have the same genes, this implicates genetic factors in alcohol dependence.However the highest concordance is between an alcohol dependent female twin and her dizygotic male co twin and since dizygotic twins share less of the same genes, this indicates that environmental factors also play an important role.

Concordance Rates of Alcohol Dependence (AD) by Zygosity and Sex

Male monozygotic twins (MZ) have a higher concordance rate for AD than dizygotic twins (DZ), suggesting a genetic effect in this instance. This was not fund for female DZ twins. The closest resemblance between twins in terms of alcohol dependence is observed for DZ twins, between a female alcoholic and her male twin (0.57 concordance). This exceeds the concordance between either male or female MZ twin pairs, suggesting a large environmental effect in this instance. 291 twin pairs from the St. Louis, MO metropolitan area were included in the sample. Zygosity was determined from detailed questionnaires, color photos, and blood grouping if necessary. Alcohol dependence was determined using a computer algorithm to integrate diagnostic ratings subsequent to subject interviews with clinicians blind to zygosity, cotwin status, and psychometric data. (Constructed from data in Prescott, C.A. et al. Am J Medical Genetics Part B (Neuropsychiatric Genetics), 134B: 48 - 55, 2005) ©2008 NBEP
Sexual abuse increases alcohol problems as well as problems with other drugs and depression.

Childhood Sexual Abuse Increases Alcohol Problems, Illicit Drug Use and Depression

Data from 1099 women interviewed in 1991 National Survey. Childhood sexual abuse was any unwanted intrafamilial sexual activity before age 18 or extrafamilial unwanted sexual activity before 18 or occurred before 13 if other person was more than 5 years older (Constructed from data in Wilsnack, S.C. et al. J Stud Alcohol, 58: 264, 1997) ©2008 NBEP
Early life stress increases alcohol intake later in life in both rhesus monkeys and humans and this is most evident in those with the l/s or s/s variant of the serotonin transporter gene.

Both genetic Factors and Adverse Early Life Experience Increase Alcohol Consumption Later in Life.

A genetic variation in serotonin transporter gene that reduces function, produces increased alcohol drinking which is most evident in both monkeys and humans who have had adverse early life experience. Compared to the I/I (long/long genotype), the s/s (short-short) and l/s genotypes of the serotonin transporter gene demonstrate less serotonin reuptake activity.A. From a study of 32 adolescent female rhesus monkeys, 18 had the mother present in a social group with other adults, 14 never had the mother or an adult present (raised first 32 days in isolation, then placed with 3 other age mates with no adult present).(Constructed from data in Barr, C.S. et al. Arch Gen Psych, 61: 1146, 2004)B. From a study of 127 children (mean age 12.5 years), 76 with maltreatment resulting in the child being removed from parental care and having 96 hour temporary custody by the state, and 51 community controls.Interviewed 2 years after entering the study.(Constructed from data in Kaufman, J. et al. Biol Psych, 61: 1228, 2007)©2007 NBEP