Major Depression and Genetics
How common is major depression? At least 10% of people in the U.S. will experience major depressive disorder at some point in their lives. Two times as many women as men experience major depression.
How do we know that genes play a role in causing depression? Scientists look at patterns of illness in families to estimate their “heritability,” or roughly what percentage of their cause is due to genes. To do this we find people with the disease who have a twin, and then find out whether the twin is also ill.
Identical (monozygotic) twins share 100% of their genes, while non-identical (“fraternal” or dizygotic) twins share 50% of their genes. If genes are part of the cause, we expect a patient’s identical twin to have a much higher risk of disease than a patient’s non-identical twin. That is the case for major depression.
Heritability is probably 40-50%, and might be higher for severe depression.
This could mean that in most cases of depression, around 50% of the cause is genetic, and around 50% is unrelated to genes (psychological or physical factors). Or it could mean that in some cases, the tendency to become depressed is almost completely genetic, and in other cases it is not really genetic at all. We don’t know the answer yet.
We can also look at adoption studies, to see whether an adopted person’s risk of depression is greater if a biological parent had depression. This also seems to be the case.
What about non-genetic factors? There are probably many non-genetic factors that increase risk of depression, many of which are probably not yet known. Severe childhood physical or sexual abuse, childhood emotional and physical neglect, and severe life stress are probably all risk factors. Losing a parent early in life probably also increases risk to some extent.
If someone has a family history of depression, are they at very high risk? If someone has a parent or sibling with major depression, that person probably has a 2 or 3 times greater risk of developing depression compared with the average person (or around 20-30% instead of 10%).
The situation is a little different if the parent or sibling has had depression more than once (“recurrent depression”), and if the depression started relatively early in life (childhood, teens or twenties).
This form of depression is less common – the exact percentage of the population is not known, but is probably around 3-5%.
But the siblings and children of people with this form of depression probably develop it at a rate that is 4 or 5 times greater than the average person.
Is there a “depression gene”? Some diseases are caused by a single defective gene. Cystic fibrosis, several kinds of muscular dystrophy, and Huntington’s disease are examples. These are usually rare diseases.
But many common disorders depression, diabetes and high blood pressure are also influenced by genes. In these disorders, there seem to be combinations of genetic changes that predispose some people to become ill.
We don’t yet know how many genes are involved in depression, but it is very doubtful that any one gene causes depression in any large number of people.
So no one simply “inherits” depression from their mother or father. Each person inherits a unique combination of genes from their mother and father, and certain combinations can predispose to a particular illness.
How are major depression and bipolar disorder related? Most people who suffer from depression do not have episodes of mania. We use the term major depression for depression without mania. Most people who experience mania also have major depression. We use the term bipolar disorder (or manic-depression) for this pattern.
Major depressive disorder and bipolar disorder are the two “major mood disorders.” For more information on the symptoms of mania abd bipolar disorder, see the links at the bottom of this page.
Most people with major depression do not have close relatives with bipolar disorder, but the relatives of people with bipolar disorder are at increased risk of both major depression and bipolar disorder.
What about major depression and anxiety disorders? There are probably genetic changes that can increase the predisposition to both major depression and to certain anxiety disorders including generalized anxiety disorder, panic disorder and social phobia.
Also, some people have a more general lifelong tendency to experience unpleasant emotions and anxiety in response to stress.
Psychologists use terms “neuroticism” and “negative affectivity” to refer to this tendency, and people who have it are also more ly to experience major depression.
However, many people who develop major depression did not have this type of personality before their depression started.
The Relationship Between Humor and Depression
People's experiences of humor vary, and humor serves many different functions in our daily lives. Sometime we use humor as a coping mechanism when things are not going well, and other times we enjoy a good laugh with our friends.
Researchers use various scales to assess our experiences of humor, but by far the most common one is the Humor Styles Questionnaire.
The scale, developed by Rod Martin and his students, is used to assess how people use humor in their daily life and has been used in hundreds of studies.
People are asked to rate how much they agree or disagree with various statements (e.g., “I enjoy making people laugh.”). Their answers create a score on four humor styles. The styles are:
Affiliative Humor: Tendency to share humor with others, tell jokes and funny stories, make others laugh, use humour to facilitate relationships, put others at ease.
Self-Enhancing Humor: Tendency to maintain a humorous outlook on life even when not with others, use humor in coping with stress, cheer oneself up with humor.
Aggressive Humor: Tendency to use humor to disparage, put down, or manipulate others; use of ridicule, offensive humor; potentially use sexist and racist jokes.
Self-Defeating Humor: Tendency to amuse others at one’s own expense, self-disparaging humor; laughing along with others when being ridiculed or teased; using humor to hide one’s true feelings from self and others.
You can take the test for yourself and see how you score on each of the four humor styles.
A recent study looked at the relationship between these styles and depression. Depression is a serious mental illness that affects millions of people around the world.
Scoring high on the two positive humor styles has been linked with various positive health outcomes, such as being happier and having healthier relationships.
On the other hand, having high scores on the negative humor styles can have a negative effect on one's health.
The current study looked at the relationships between the four humor styles and depression. The researchers wanted to test whether people diagnosed with depression score high on the two negative humor styles, and low on the two positive styles. In addition, they looked at whether there is a genetic underpinning to the relationship between humor styles and depression.
To do so, they recruited a sample of 1,154 Australian twins. Three hundred thirty-nine of them were identical, and 236 were non-identical. All were same-sex twins, with both male and female pairs represented. Of the whole sample, 145 individuals were diagnosed with depression.
All twins completed the Humor Styles Questionnaire and in addition, answered three questions pertaining to depression. These questions were taken from different scales and are in line with items used to diagnose depressive disorder, though no official diagnosis was performed.
The results of the study showed first, that women were two and half times more ly than men to suffer from depression, a fact that has been found in other studies. Second, as predicted, people diagnosed with depression used self-defeating humor more than non-depressive people.
In addition, depressive twins used the two positive humor styles (affiliative and self-enhancing) less then non-depressive twins, also as hypothesized. Contrary to the prediction, there were no differences in the use of aggressive humor among the depressive and non-depressive twins.
Next, the researchers looked at the possible genetic and environmental factors that may account for these correlations.
Comparisons between identical and non-identical twins allow us to extract the relative contributions of genetic and environmental factors to a given trait or correlation by using behavioral genetics tools.
Such analyses revealed that 26% of the differences in the depression scores were attributed to additive genetic factors, while 74% were due to non-shared environments (e.g., the twins being in different classrooms, having unique friends). Shared environments (e.g., raised by the same family) had 0% influence.
Other results showed that the positive correlation between the depression scale and self-defeating humor, and the negative correlation between the depression scale and self-enhancing humor, each have an underlying genetic factor influencing the correlations.
In sum, the study not only established that humor styles are correlated with depression, but also that at least some of these correlations are the result of some underpinning genetic influences (though not identifiable yet). The research also highlights the fact that humor is not always good for us, and in some cases might have severe negative effects.
Correlations, of course, do not imply causation, and it is unclear what they mean exactly.
Do people suffering from depression choose to use more negative styles of humor and not try to improve their situation by using a more positive humor style? Or perhaps using a negative humor style is influencing depression? The third and ly possibility is that certain factors (both genetic and environmental) affect depression and the use of negative styles, but more research is needed on this topic.