Teens who are more likely to argue aggressively and persistently with their parents also have brain structures of different sizes, a new study finds. On average, a sassy kid has a larger amygdala, the part of the brain that processes emotional responses, than a cooperative one. The findings could help scientists understand the roots of aggressive behavior in teens.
For teenagers, the battle begins within. When hormones surge in puberty, the amygdala grows in size and becomes more active, leading to rash and emotional behavior. Like a chaperone, the prefrontal cortex regulates the amygdala to ensure socially correct behavior. But during adolescence, it still has maturing to do, scientists speculate, so the amygdala is left poorly supervised. As a person grows into adulthood, inhibitory connections between these two regions increase and the prefrontal cortex catches up with the amygdala. This structural change--the growth of the prefrontal cortex and shrinking of the amygdala--could account for less impulsive behavior in adulthood.
Nicholas Allen, a psychologist at the University of Melbourne, Australia, wanted to find out whether brain structure influences a teenager's day-to-day emotional state. He and colleagues from the Orygen Research Centre in Melbourne and the Oregon Research Institute in Eugene videotaped 137 preteens and teenagers between the ages of 11 and 14 as they talked with their parents about issues that often lead to disagreements--such as bedtime, homework, or cell-phone use. The researchers scored the conversations based on their content and other factors such as facial expressions and tone. They also used brain imaging to measure the volume of different brain structures among the youths. Those with a larger amygdala, relative to the total brain size, showed more aggressive behavior while talking with their parents in these sessions.
In addition, boys with a left prefrontal cortex larger than the right were less emotionally reactive, the researchers report online this week in the Proceedings of the National Academy of Sciences. Allen speculates that the left side of the cortex plays a greater role in squelching impulsive behavior, meaning that when it's larger than the right side, impulses may be better controlled.
The study doesn't show that a larger amygdala causes the behavior differences, but it's a good start toward spelling out the connection, says psychiatrist Jay Giedd of the National Institute of Mental Health in Bethesda, Maryland. Researchers have been seeking a connection "between the brain imaging and the 'so what,' or the clinical significance," he says, but that link "has been elusive." The next step is to track these adolescents and see how their brains change over time, Giedd says: "It is the right approach; there is more to come."