Drug and alcohol, and even smoking cigarettes, don’t mix with getting a college degree. Those who started alcohol or drug use by age 14, or who became dependent on nicotine, drinks or marijuana, were less likely to finish college than those who took up vices later on or never did, a new study finds.

The research, detailed in the August issue of Alcoholism: Clinical & Experimental Research, involved 6,242 male twins who served in the military during the Vietnam era. It was conducted by researchers at Washington University School of Medicine in St. Louis and the Palo Alto Veterans Affairs Health Care System. The men in the study were surveyed when most were in their late 30s of early 40s.

“We can't say that substance dependence or early substance use causes lower educational achievement, but we do see a strong association,” said lead author Julia D. Grant. “Even after we statistically controlled for the genes and the environmental factors that twins share, we found a relationship between substance use and educational achievement.”

“Studying identical and fraternal twin pairs is useful for examining things like substance use and education because we can asses the extent to which a given behavior is influenced by genetic factors and by factors related to family and environment,” Grant said. “Since identical twins share all of their genes and fraternal share about half, we can set up statistical comparisons to tease many of those factors apart.”

Because of the G.I. Bill, veterans are less likely to have financial constraints that would prevent them from attending college, making them an interesting group for this study.

“Drugs and alcohol affect many lifetime milestones such as marriage, parenthood and employment, which are closely linked to education,” Grant said. “These events in later life all are influenced by early substance use, and this study provides further evidence that as a society, we need to continue our public-health efforts to reduce underage drinking, smoking and use of drugs.”