Although children can be depressed for many reasons, new evidence suggests that there are physiological differences among depressed children based on their experiences of abuse before age 5. Early abuse may be especially damaging due to the very young age at which it occurs.

Those are the findings of a new study of low-income children that was conducted by researchers at the University of Minnesota and the University of Rochester, Mt. Hope Family Center. The study appears in the January/February 2010 issue of the journal Child Development.

Children who experience maltreatment, including physical, sexual, and emotional abuse or neglect, grow up with a lot of stress. Cortisol, termed the "stress hormone," helps the body regulate stress. But when stress is chronic and overloads the system, cortisol can soar to very high levels or plummet to lows, which in turn can harm development and health.

The researchers studied more than 500 low-income children ages 7 to 13, about half of whom had been abused and/or neglected, to find out whether abuse early in life and feelings of depression affected their levels of cortisol. High levels of depression were more frequent among children who were abused in the first five years of their lives than among maltreated children who weren't abused early in life or children who weren't maltreated at all.

More importantly, only children who were abused before age 5 and depressed had an atypical flattening of cortisol production during the day, whereas other children, whether they were depressed or not, showed an expected daily decline in cortisol from morning to afternoon. This finding means that the body's primary system for adapting to stress had become compromised among children who were depressed and abused early in life. The results suggest that there are different subtypes of depression, with atypical cortisol regulation occurring among children who were abused before age 5.

The authors suggest that early abuse may be more damaging to developing emotion and stress systems because it happens as the brain is rapidly developing and when children are more dependent on caregivers' protection. Moreover, because it's harder for very young children to discern the clues predicting an abusive attack, they may be chronically stressed and overly vigilant, even when they're not being abused.

"In the United States, more than 1.5 million children are abused and neglected every year, though it's estimated that the actual rates are substantially greater," according to Dante Cicchetti, McKnight Presidential Chair and professor of child development and psychiatry at the University of Minnesota, who led the study.

"The results of this study have significant implications for children in the child welfare population and underscore the importance of providing early preventive interventions to children who have been abused."