SUNDAY, Aug. 11, 2013 (HealthDay News) -- Teens who have half-siblings -- brothers or sisters with a different father -- are more likely to use drugs and have sex by age 15 than teens with only full siblings, according to a new study.
Although this family scenario isn't new, researchers say it's becoming more common as a growing number of unmarried people have children.
In conducting the study, researchers from Bowling Green State University and Iowa State University examined how "multi-partnered fertility" -- re-partnering and having more children -- affects children's drug use and sexual behavior.
"We find that first-born adolescents with half-siblings with the same mother but a different father do have less favorable outcomes compared to their peers with only full siblings, even after accounting for the mother's background characteristics, socioeconomic factors the child experienced growing up, and family instability and structure," said researcher Karen Benjamin Guzzo, an assistant professor of sociology at Bowling Green.
The study's findings were scheduled for presentation Sunday at the annual meeting of the American Sociological Association in New York City. Data and conclusions presented at meetings are typically considered preliminary until published in a peer-reviewed medical journal.
For first-born children, multi-partnered fertility often means enduring the breakup of their biological parents and living for a time in a single-mother household. It could also mean experiencing their mother finding at least one new partner, possibly living with a stepfather and watching their mother have another child with someone other than their father.
Although the study focused on mothers and their first-born children who lived with their mother for most of their lives, the researchers also considered the mother's level of education, household poverty and the number of changes in family structure the teens faced, such as whether their mother ever married or lived with their father or another partner.
The study showed that by age 15, teens who have a half-sibling with a different father are roughly 65 percent more likely to have used drugs, including marijuana, stimulants, inhalants, cocaine, crack, hallucinogens and sedatives. These teens were also about two and a half times more likely to have had sex by the time they turned 15.
The reasons for this association between risky teen behavior and having half-siblings remains unclear, the researchers said in an association news release. They hope to do more research to determine if younger siblings react in the same way as first-borns, and to explore differences in how teens view their relationship with their mother.
Although the study showed an association between having half-siblings and an increased likelihood of using drugs and having sex as a teen, it did not prove a cause-and-effect relationship.
The U.S. Centers for Disease Control and Prevention provides more information on teen alcohol and drug use.