Biological explanations

of Depression

Genetic explanationFamily, twin and adoption studies

Mood Disorders – Genetic

• Depression tends to run in families

• The closer the relationship with a sufferer, the more likely a person will have a mood disorder

• Genetic evidence is strongest for most severe forms of the disorder

• Concordance rates also indicate a substantial environmental contribution

Mood Disorders – Genetic

• Zubenko et al (2001)– Family history – 50% of FD relatives, 25% of

SD relatives also had mood disorder– Relatives had increased risk of suicide & liver

disease

Genetics & Depression

Qu. How do you ‘prove’ a genetic

argument for any behaviour?Twin

studies:

MZ

DZ

Allen, 1976

40%

11%

Bertelsen 1977

59%

30%

Qu. Problems?

Genetics & DepressionFamily concordance

rates

Gen Pop

Gershon 1990

15%

40-50%

Wender 1986

15%

70% if adopted

Qu. Problems?

Family

Genetic explanationFamily studies

Gershon

DEPRESSIONRESEARCHER

“I showed that depression rates in these first degree relatives was about two to three times the rate in the general

population …

“In 1990, I reviewed many family studies …

… in each study, depression was assessed

in the first degree relatives of patients with

depression …

“My work was supported by Sullivan et al (2000), who showed

similar findings in a review of several family studies…

Genetic explanationTwin studies

McGuffin 1996

“In twin studies, we look at CONCORDANCE RATES for depression in MZ and

DZ twins …

MZ twins have identical genes!If depression is caused by genes, then if they have the gene they should BOTH have depression!

DZ twins share 50% of their genes!

So one twin could have the gene for depression, while the other doesn’t!

“If depression is caused by a

gene, we would expect that MZ

twins either BOTH have

depression, or NEITHER have depression …

“So we would expect to find a

HIGHER CONCORDANCE

for depression in MZ twins than in

DZ twins …

Genetic explanationTwin studies

McGuffin

“Here are the twin study research findings …

MZ: either both have the gene for depression, or neither does – so should be concordant

DZ: might both have the gene, both not have the gene, or one could have the gene and one might not have it! This means we expect a lower concordance for depression in DZ twins, than we do in MZ twins

“In 1976, Allen reviewed MZ/DZ

studies and found a mean

concordance rate of 40% for MZ twins, and just

11% for DZ twins …“In 1977,

Bertelson et al found a

concordance of 59% for MZ twins,

and 30% for DZ twins …

“In 1996, my team and I found 46% concordance for depression in

MZ twins, and just 20% in DZ

twins …

Genetic explanationTwin studies

McGuffin

“I’ll summarise the findings for you …

CONCORDANT FOR DEPRESSION?

CONCORDANT FOR DEPRESSION?

TWIN STUDIES …Allen (1976) MZ

40% DZ 11%

Bertelson (1977) MZ 59%

DZ 30%

McGuffin (1996) MZ 46%

DZ 20%

“Do these data support the idea

that depression is caused by genes?

…

“Thinking about what you

understand about genes and MZ/DZ

twins, can you criticise these

findings? …

Genetic explanationAdoption studies

Wender

“I looked at adoption studies, because when a child is adopted, it is

taken away from its home and family environment …

“Family studies like Gershon’s, and twin studies like McGuffin’s,

can’t say genes are the only reason for depression, because they didn’t separate genes from

environment …

“Adoption studies are an

improvement, because they eliminate the

influence of the family

environment …

What causes depression

Genetic explanationAdoption studies

Wender

“Both sets of findings suggest

that genetics play a big part

in depression …

“I published two sets of

findings in 1986 …

We found that biological relatives of adopted sufferers of depression were SEVEN TIMES more likely than adoptive relatives to have had depression themselves!When we looked at adopted children who later developed depression, we found that their biological parents were EIGHT TIMES more likely than their adoptive parents, to have suffered from depression

I’m feeling prickly today…

So am I … but I don’t know why!

And we’re happy … so why is our

adopted lad depressed???

We’re both okay!

What a nice day!

Biological parentParents with adopted child

Parent with biological child