A schema is a mental representation that helps us to predict and interpret our environment.  They are based on past experiences. They are the "filters" through which we see and interact with the world.  It makes sense that this would have an effect on depression;  if we have negative schema about ourselves or our world, then we will interpret our daily experiences through this negative filter.

Memory bias is the tendency to recall negative information.  In the case of testing done on depressed individuals, it is found that they tend to remember words with negative connotations better than those with positive connotations.

It is not possible to establish a true cause and effect relationship.  We cannot be sure if negative thinking leads to depression or whether depression leads to negative thinking.  Prospective studies are difficult to carry out - and it is not really possible to separate other variables - for example, biological or social vulnerabilities.

There are primarily two ways in which rumination has been measured.  First, Nolen-Hoeksema's theory is based on extensive work with depressive patients. Through self-reporting, she has determined that this is a common characteristic of depressed patients. In addition, research like that carried out by Farb et al indicates that there are biological indications that rumination, which takes place in the frontal lobe, may play a role in depression.