Chicken or the Egg?

From a press release about a study in the Archives of General Psychiatry:

Although numerous studies have linked alcohol problems and depression, whether the relationship is causal or whether there is a common underlying factor remains unclear, they said.

To explore the issue, they examined data from the Christchurch Health and Development study, which followed 1,055 individuals born in New Zealand for 25 years from birth.

...

In an attempt to determine causality, the researchers tested three statistical models on the data:

• One in which both disorders increased risk of the other in a feedback loop
• One in which major depression caused alcohol problems
• One in which alcohol problems caused major depression

The model that fit best was the one that assumed that alcohol problems caused major depression.

"The underlying mechanisms that give rise to such an association are unclear," the researchers said. "However, it has been proposed that this link may arise from genetic processes in which the use of alcohol acts to trigger genetic markers that increase the risk of major depression."

"In addition," they said, "further research suggests that alcohol's depressant characteristics may lead to periods of depressed affect among those with alcohol abuse or dependence."

Stress related to social, financial, and legal problems stemming from alcohol use might also increase the risk of depression, they said.