Another explanation could be that people who are depressed are more likely to keep a diary. This is an observational study, so it is not possible to associate that writing in a diary as a cause, leads to depression as the effect.Their interpretation of the results was flawed because the researchers assumed an order of cause and effect and looked at the results only from one direction. Another example would be if we assume that obesity is related to depression. We can look at it two ways: that depression can lead to obeisty through over eating or the other way around, that overweight people are more likely to become depressed.
