We all have our ups and downs — a fight with a friend, a divorce, the loss of a parent. But most of us get over it. Only some go on to develop major depression.

In fact, major depression is one of the most common mental disorders in the United States. The Centers for Disease Control and Prevention (CDC) reports that more than 1 out of 20 Americans 12 years of age and older reported current depression from 2005-06.

According to the World Health Organization (WHO), major depression also carries the heaviest burden of disability among mental and behavioral disorders. Nearly 8 percent of persons 12 years or older (6 percent of males and 10 percent of females) report current depression. Females have higher rates of depression than males in every age group. Males aged 40-59 years have higher rates of depression, 7 percent, than males aged 60 years or older, 5 percent. Females aged 40–59 years have higher rates of depression than females aged 12-17 years and females 60 years or older

People with depression or healthy people with a depressed mood can be affected by depressive thoughts. However, according to a recent study, depressive thoughts are maintained for longer periods of time for people with a depressed mood, and this extended duration may reduce the amount of information that these individuals can hold in their memory, according to Center for BrainHealth principal investigator Dr. Bart Rypma, who is also Meadows Foundation chair at the University of Texas at Dallas.

Although it has been known that negative thoughts tend to last longer for those with depression, this study is unique in showing that these thoughts, triggered from stimuli in the environment, can persist to the point that they hinder a depressed person's ability to keep their train of thought.

In this study, researchers recruited 75 university undergraduate students. Thirty students were classified as having depressive symptoms, and 45 participants were categorized as not exhibiting depressive symptoms.

All participants were asked to respond to a sentence featuring depressive thoughts, such as "I am sad," "People don't like me," or neutral information. They were then asked to remember a string of numbers.

Those with a depressed mood forgot more number strings than those without depressed mood when responding to a sentence with negative information. Participants with depressed mood who were given the depressive thought first, remembered 31 percent fewer number strings compared to participants who did not have a depressed mood, and those with a depressed mood who were given the number string first.

The study's lead author, Nick Hubbard, a doctoral candidate at the Center for BrainHealth, explained that we all have a fixed amount of information we can hold in memory at one time. The fact that depressive thoughts do not seem to go away once they enter memory may explain why depressed individuals have difficulty concentrating or remembering things in their daily lives.

This preoccupation of memory by depressive thoughts might also explain why more positive thoughts are often absent in depression. because there simply is not enough space for them. Importantly, this greater dedication of memory resources to depressive thoughts and an impoverished ability to hold positive thoughts in memory, might be the key to understanding how depression develops and continues throughout an individual's lifespan.

Interventions such as mindfulness-based cognitive therapy are quite successful in empowering depressed people to recognize and better regulate the content of their thoughts. The researchers hope to continue studying how such therapeutic approaches can alter the depressed brain and how these alterations might result in better memory and outcomes for persons with depression.