Lecture 8: Psychopathology in humans and nonhuman primates – why do humans get depressed?

Depression in humans

• Feelings of sadness / emptiness
• No interest or pleasure in activities
• Weight changes
• Insomnia or hypersomnia
• Agitation or retardation
• Fatigue or loss of energy
• Feelings of worthlessness
• Poor concentration
• Recurrent thoughts of death

Evolution an depression

• Depression is common
• High heritability
• People with depression have less children
• Paradox: Why did natural selection not eliminate depression from the population?

Prevalence and heritability

• Lifetime prevalence (USA): 19.2%,  (NL): 17.9%
• University students: 30.6%
• Persistent depressive disorder (USA): 4.7%
• Heritability = 37%

Why do humans get depressed?

• Cry for help
• Reaction to losses
• Seasonal – no activity in winter
• Energy conservation
• Rumination – solving problems
• Association with inflammation/infections
• Mismatch

Mismatch hypothesis

• We are not adapted to modern environments, but to ancestral environments. This results in a mismatch.

Testing mismatch hypothesis -test 1

Chimpanzees in the wild – less mismatch

Captive chimpanzees – more mismatch

Captive chimpanzees were more likely to show signs of depression:

• Depressed hunched posture
• Social withdrawal
• Easily irritated or angered
• Loss of interest in activities
• Weight loss or gain
• Awake or easily awakened
• Difficulty falling asleep or excessive sleep
• Poor concentration

Environment of Evolutionary Adaptedness

• Savannah-like environment
• Hunter-gatherer society
• Nomadic lifestyle
• Physical dangers
• High infant mortality
• Food scarcity

Mismatch hypothesis – test 2

Developing countries – less mismatch

Developed countries – more mismatch

• It was found that the higher the GDP per capita is somewhere, the more people suffer from mood disorders

Key environmental changes

• Obesity
• Different diet
• Less physical activity
• Less sunlight
• Less sleep
• Less social interactions

Mismatch hypothesis – test 3

Rural areas – less mismatch

Urban areas – more mismatch

High prevalence of diagnoses in Stockholm, Sweden, compared to Swedish rural areas:

• Depression
• Anxiety
• Diabetes
• Hypertension
• Asthma
• COPD

Mismatch hypothesis – test 4

Hunter-gather societies – less mismatch

Modern societies – more mismatch

• No signs of depression were found in Kaluli, a hunter-gather society in New Guinea

Mismatch hypothesis – test 5

Co-sleeping babies – less mismatch

Babies sleeping alone – more mismatch

When co-sleeping:

• Less infant crying
• Less sudden infant death syndrome
• Higher self-esteem
• Less anxiety
• More satisfaction with life
• More social competence

Mismatch hypothesis – test 6

More physical activity – less mismatch

Less physical activity – more mismatch

Prospective study in adolescents:

• Each additional hour of physical activity per week - reduction in depressive symptoms of 8%

Mismatch hypothesis – test 7

More mismatches – more depression

Less mismatches – less depression

Mismatch questionnaire with scales:

• Lack of sleep
• Lack of exercise
• Lack of natural daylight
• Lack of time
• Lack of green environment
• Lack of social relations
• Lack of contact with family members
• Materialism

Mismatch correlation with depressive symptoms (BDI): 0.6

Conclusion

Modern humans behave like captive apes

Solution

Do we all need to become hunters and gatherers again...?

Mindfulness: Non-judgmental and accepting awareness of the present moment

• Mindfulness improves mental health
• It prevents depressive relapse

General conclusion

An evolutionary informed psychiatry and developmental psychology may improve mental health and children’s education