Lisa N. LeGrand, et al, in American Scientist:
In 1994, the 45-year-old daughter of Senator and former presidential nominee George McGovern froze to death outside a bar in Madison, Wisconsin. Terry McGovern’s death followed a night of heavy drinking and a lifetime of battling alcohol addiction. The Senator’s middle child had been talented and charismatic, but also rebellious. She started drinking at 13, became pregnant at 15 and experimented with marijuana and LSD in high school. She was sober during much of her 30s but eventually relapsed. By the time she died, Terry had been through many treatment programs and more than 60 detoxifications.
Her story is not unique. Even with strong family support, failure to overcome an addiction is common. Success rates vary by treatment type, severity of the condition and the criteria for success…
Studies of twins are particularly useful for analyzing the origins of a behavior like addiction. Our twin pairs have grown up in the same family environment but have different degrees of genetic similarity. Monozygotic or identical twins have identical genes, but dizygotic or fraternal twins share on average only half of their segregating genes. If the two types of twins are equally similar for a trait, we know that genes are unimportant for that trait. But when monozygotic twins are more similar than dizygotic twins, we conclude that genes have an effect.
More here.