Kids with behavior problems became sexually active earlier than their peers, in a recent study.

Having sex early - before age 16 - increases the risk of teen pregnancy, partner violence, sexually transmitted infections and other negative health outcomes, the researchers say.

Other studies have found that behaviors like frequent opposition or rule breaking are associated with earlier age of first sexual intercourse or teenage pregnancy, said lead author S. Rachel Skinner of the University of Sydney in Australia, in email to Reuters Health.

The new study is larger and more robust, she said.

“We were able to disentangle this relationship from many other known risk factors, and demonstrate that (behavior problems are) an independent predictor of earlier age of first sexual intercourse,” Skinner said.

Her team used data from a long-term study beginning in 1989 that followed almost 3,000 Australian children and their families from birth through age 17.

By age 17, 44 percent of boys and 50 percent of girls said they’d had intercourse.

Overall, almost 22 percent of boys and 25 percent of girls said they started before age 16, usually around age 15.

Parents answered questions about their child’s “internalizing” behavior, like withdrawal, physical complaints, anxiety and depression, and “externalizing” behavior, like aggression and criminal behavior.

Kids with total behavioral scores high enough to be considered problematic were more likely than other kids to have sex before age 16.

High scores on externalizing behavior were associated with having sex before age 16 for both boys and girls, but internalizing behavior was related to age at first intercourse only for boys, according to results in Pediatrics.

Bad behavior starting at age five for boys and age 10 for girls increased the likelihood of having sex before age 16 when the authors accounted for other factors like parental age, education attainment and socioeconomic status.

Researchers can only speculate about why externalizing behavior and early sexual activity would be connected, Skinner said.

“We do know that young people who display externalizing behavior are more likely to take risks, and to be sensation seekers,” she said. “They are more likely to take physical risks, for example, while riding motorbikes or skateboarding.”

Having sex earlier could be part of a pattern of risk-taking, she said.

“Delinquent behavior stems from a child not understanding or not caring about the consequences that will result,” said Patricia A. Cavazos-Rehg, assistant professor of psychiatry at Washington University School of Medicine in St. Louis, Missouri.

Cavazos-Rehg, who was not involved in the new research, told Reuters Health by email, “It does not matter what the behavior is, if the child does not understand or care about consequences, then he or she will most certainly engage in a deviant behavior if temptation should arise. An individual with self-regulation problems as a child is on a trajectory to have these problems as a teen unless someone or something intervenes.”

For both genders, younger age of the mother, lower level of maternal education, the father of the child not living with the mother and parents who smoke or use drugs increased the risk for early sexual intercourse.

“Most of these factors are not changeable by any intervention that we know works,” Skinner said. “It is probably better for us to use our understanding of the risk factors of early (first sexual intercourse) to identify those who are vulnerable and make sure they receive support to stay at school, to have goals of a career or work after school, and to ensure they receive well designed sexual health and relationships education from a young age.”