There are certain aspects of the disease that may be measured in animals. One of the core symptoms of depression is anhedonia, the decrease and loss of interest in pleasurable activities. We measure interest in food that animals like a lot or in motivation for sexual activity. We also measure how they are interacting socially with other animals in the group, and changes in sleep patterns and daytime activities. Another behavior that has been used frequently to measure animal depression is whether they readily give up when exposed to a stressful situation.
Negative thinking. People with depression get stuck in negative thinking. This can make people focus on problems and faults. It can make things seem bleaker than they really are. Negative thinking can make a person believe things will never get better, that problems are too big to solve, that nothing can fix the situation, or that nothing matters.
Negative thinking can be self-critical, too. People may believe they are worthless and unlovable — even though that's not true. That can lead people with depression to think about harming themselves or about ending their own life. Negative thinking can block our ability to see solutions or realize that a problem is actually temporary.