Researchers reported that for men,being unmarried increases the risk of depression.These findings were based on the medical records of 400 married men and 500 unmarried men.Why is this conclusion not justified?

Since there is no random assignment,there is no way to know that being unmarried increases the risk of depression; there may have been lurking variables.For example unmarried men may be on average less well off,younger,more (or less)educated.Maybe the rate of unemployment is higher amongst unmarried men (those who are unemployed may find it harder to find a wife).