Health professionals used to differentiate between “reactive” and “biological” depressions – the former felt to be due to stress, and the latter, to heredity. There is no longer felt to be a meaningful difference. Trying to separate the two is like trying to decide whether the chicken or the egg came first. As with many conditions in psychiatry, depression is felt to be due to a combination of “Nature and Nurture”- the combination of genetics and environment. The stronger the inherited (genetic) tendency to depression, the less stress is required to trigger a depressive episode. Genes are the building blocks of the body. They tell the body how to arrange itself, as an architect’s blueprints tell how to construct a house.
Picture yourself sitting on a mat made of twigs covering a dirt pit. The mat is a family heirloom, passed down from generation to generation. Suddenly, monkeys start throwing coconuts at you. The mat symbolizes your resistance to depression. Coconuts symbolize stressful life events. The mat weakens each time a coconut (stressor) smashes onto it. If the mat breaks, you fall into the pit of depression. Splat! Your best friend moved away. Splat! Your dog died.
Splat! Your parents divorced. A strong inherited mat can withstand several coconuts without
breaking. A weaker mat will take fewer coconuts to break. Of course, if you are very lucky, you inherited a cement mat, and no amount of coconuts will break it-you will not get depressed no matter how much stress you experience. Even if you have a weak mat, you may still be lucky and avoid the pit by not being hit with any coconuts. If you are very unlucky, your mat is very weak, and will break without any coconuts dropping on it-you will get depressed despite no major stressful life events. Treatment for depression is the ladder you build that allows you to climb out of the pit. Psychotherapy techniques are like expert courses in mat making. They teach you the tools to prevent your depression from returning.
A New Zealand study from 2003 may help explain the how the combination of genetics and stress leads to depression. Scientists found that differences in the gene that controls serotonin transportation in the brain were associated with vulnerability to depression. The body carries two copies of each gene, one from each parent. There can be either a “long” or “short” version of the serotonin transporter gene. This gene produces the protein that recycles serotonin from the nerve synapse back into the presynaptic nerve. The short version of the gene makes a less efficient serotonin transporter protein.
The scientists tracked the number of stressful life events of 847 people between the ages of 21 and 26 in 847 people. They found that people who had two “long” versions of the gene (31%) were much less likely to develop depression even if they had gone through many stressful events-only 17% developed depression. People with two copies of the “short” version of the gene (17% of the study population) had the highest incidence of depression when under stress- 43% developed depression. People with one short and one long version of the gene (51%) had an intermediate incidence of depression-33% became depressed (Caspi, et al., Science, July 2003). The “long” versions of the gene are like the strong “mats” that protect against falling into the “pit” of depression.
A common way to determine how much of a condition is inherited and how much is due to the environment is with twin studies. The scientific term for identical twins is monozygotic-they come from the same fertilized egg and share the same genes. Fraternal twins are dizygotic-they come from two different fertilized eggs and share only half of the same genes, the same as if they were regular siblings. (The most information can be obtained from identical twins who were raised apart from each other. Since they have the same genes, any differences between them are due to the environment.) Heritability is calculated by analyzing the difference in frequency that a trait is seen in monozygotic vs. dizygotic twins. The bigger the difference, the bigger the contribution of inheritance.
Suppose that I’ve discovered a trait called Bubblitis-laughing uncontrollably when a soap bubble lands on your nose. I would like to see how much of Bubblitis is due to inheritance, and how much is due to environmental factors. I study groups of twins, and find that if one twin pair has Bubblitis, the other twin also has it in 100% of the cases when the twins are identical.
However, for fraternal twins, if one twin pair has Bubblitis, the other twin has it in only 50% of the cases. This tells me that Bubblitis is strongly inherited, with virtually no influence from the environment. However, if identical twins and fraternal twins both have almost the same incidence of Bubblitis, this tells me that Bubblitis is most likely due to environmental influences with virtually no genetic contribution. Perhaps when the twins were babies, their parents laughed as they blew bubbles onto their noses, teaching them that bubbles on the nose were funny.
A Swedish study found that heritability of major depression was higher for women (42%) than men (29%). However, environmental factors still contributed more than half the risk for depression (Kendler, Kenneth et al., American Journal of Psychiatry, Jan 2006).
Is depression inherited or is it due to stress? Health professionals used to differentiate between “reactive” and “biological” depressions- the former felt to be due to stress, and the latter, to heredity. There is no longer felt to be a meaningful difference. Trying to separate the two is like trying to decide whether the chicken or the egg came first. As with many conditions in psychiatry, depression is felt to be due to a combination of “Nature and nurture”- the combination of genetics and environment. The stronger the inherited (genetic) tendency to depression, the less stress is required to trigger a depressive episode. Genes are the building blocks of the body. They tell the body how to arrange itself, as an architect’s blueprints tell how to construct a house.
Picture yourself sitting on a mat made of twigs covering a dirt pit. The mat is a family heirloom, passed down from generation to generation. Suddenly, monkeys start throwing coconuts at you. The mat symbolizes your resistance to depression. Coconuts symbolize stressful life events. The mat weakens each time a coconut (stressor) smashes onto it. If the mat breaks, you fall into the pit of depression. Splat! Your best friend moved away. Splat! Your dog died.
Splat! Your parents divorced. A strong inherited mat can withstand several coconuts without breaking. A weaker mat will take fewer coconuts to break. Of course, if you are very lucky, you inherited a cement mat, and no amount of coconuts will break it-you will not get depressed no matter how much stress you experience. Even if you have a weak mat, you may still be lucky and avoid the pit by not being hit with any coconuts. If you are very unlucky, your mat is very weak, and will break without any coconuts dropping on it-you will get depressed despite no major stressful life events. Treatment for depression is the ladder you build that allows you to climb out of the pit. Psychotherapy techniques are like expert courses in mat making. They teach you the tools to prevent your depression from returning.
A New Zealand study from 2003 may help explain the how the combination of genetics and stress leads to depression. Scientists found that differences in the gene that controls serotonin transportation in the brain were associated with vulnerability to depression. The body carries two copies of each gene, one from each parent. There can be either a “long” or “short” version of the serotonin transporter gene. This gene produces the protein that recycles serotonin from the nerve synapse back into the presynaptic nerve. The short version of the gene makes a less efficient serotonin transporter protein.