Researchers have found that parents who anger easily and over-react are more likely to have toddlers who act out and become upset easily.

The research is an important step in understanding the complex link between genetics and home environment. In the study, researchers from Oregon State University, Oregon Social Learning Center, and other institutions collected data in 10 states from 361 families linked through adoption -- and obtained genetic data from birth parents as well as the children.

They followed the children at nine, 18 and 27 months of age, and found that adoptive parents who had a tendency to over-react, for example, were quick to anger when children tested age-appropriate limits or made mistakes. These over-reactive parents had a significant effect on their children, who exhibited "negative emotionality," or acting out and having more temper tantrums than normal for their age.

Genetics also played a role, particularly in the case of children who were at genetic risk of negative emotionality from their birth mothers, but were raised in a low-stress or less-reactive environment.

The study was published in the latest edition of the journal Development and Psychopathology.

"This is an age where children are prone to test limits and boundaries," said lead author Shannon Lipscomb, an assistant professor of human development and family sciences at OSU-Cascades. "However, research consistently shows that children with elevated levels of negative emotionality during these early years have more difficulties with emotion regulation and tend to exhibit more problem behavior when they are of school age."

Researchers also found that children who exhibited the most increases in negative emotionality as they developed from infants to toddlers (from nine to 27 months of age) also had the highest levels of problem behaviors at age two, suggesting that negative emotions can have their own development process that has implications for children's later behaviors.

"This really sets our study apart," Lipscomb said. "Researchers have looked at this aspect of emotionality as something fairly stable, but we have been able to show that although most kids test limits and increase in negative emotionality as they approach toddler age, the amount they increase can affect how many problem behaviors they exhibit as 2-year-olds."

Lipscomb said the take-away message for parents of young children and infants is that the way they adapt to toddlerhood -- a challenging time marked by a child's increasing mobility and independence -- can have an impact on how their child will develop.

"Parents' ability to regulate themselves and to remain firm, confident and not over-react is a key way they can help their children to modify their behavior," she said. "You set the example as a parent in your own emotions and reactions."

Researchers from the Oregon Social Learning Center, University of Pittsburgh, Pennsylvania State University, University of New Orleans, University of Minnesota, University of California, Davis and Yale Child Study Center contributed to this study, which was supported by a grant from the National Institutes of Health.