Adolescents who have half-siblings with a different father are more likely to have used drugs and had sex by age 15 than those who have only full siblings. That's according to new research from Karen Benjamin Guzzo, an assistant professor of sociology at Bowling Green State University, and Cassandra Dorius, an assistant professor of human development and family studies at Iowa State University.

Using data from the National Longitudinal Study of Youth, they examined a phenomenon known as "multi-partnered fertility" or MPF. This happens when parents who are not romantically involved with each other form new relationships and have another child with a new partner.

"It's not new behavior, but it's happening more often as more people are having children outside of marriage," said Guzzo.

According to Guzzo, this is one of the first studies to examine the effect of parental MPF on children over the long-term, and the only study that takes into account background factors (such as the mother's level of education and household poverty) and the number of changes in family structure the adolescent experienced.

The researchers looked at the connections between this re-partnering and additional childbearing on adolescent drug use and early sex. They focused on mothers and first-born children who lived with their mother most of their lives.

"For children, MPF means having a half-sibling, but it also means, for first-born children, that they usually experienced their biological parents splitting up—if they were together at all, lived in a single mother household for some time, experienced their mother finding a new partner at least once and perhaps lived with a stepfather, and finally experienced their mother having a baby with a new partner," Guzzo explained.

Researchers looked at the mother's educational background, her own family structure growing up, and whether the child experienced bouts of poverty. They also examined family factors—whether the father lived with them at birth, how many family transitions the adolescents experienced, and whether the mother ever married or cohabited, with the child's father or another partner.

"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," Guzzo said.

"Adolescents with a half-sibling with a different father are about 65 percent more likely to have used marijuana, uppers, inhalants, cocaine, crack, hallucinogens, sedatives, or other drugs by the time of their 15th birthday than those who have only full siblings. They are also about 2.5 times more likely to have had sex by their 15th birthday than their peers with only full siblings."

Guzzo said it's not clear yet what drives these outcomes, but that in the future she and Dorius plan to explore differences in maternal behaviors, father and stepfather involvement, and adolescent perceptions of their relationship with their mother to see if these factors explain the association between having half-siblings with a different father and risky adolescent behavior.

"We are also planning to look at whether this association holds for children other than the first-born, who tend to experience the most instability," Guzzo said.

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More information: The paper, "Maternal Multipartnered Fertility and Adolescent Well-being," will be presented on Tuesday, Aug. 13, at 12:30 p.m. EDT in New York City at the American Sociological Association's 108th Annual Meeting.