Translated by Rumia Bose

Is a shortage of serotonin the cause of your depression? The one sees mental disorder as a brain disease and the other as a behavioural problem. Who is right?
There is no doubt that serotonin and depression are somehow related to each other. Abnormalities in serotonin in the brains of depressed people were established more than forty years ago. These findings have been corroborated in various ways in the intervening years. If you think that I too am reducing a human to an (serotonergic) object then I will now reassure you that this is not the case. Those findings about serotonin and depression do not prove that depression is caused by abnormalities in serotonin. They prove only that there is some relation between them. The interesting question for me is: how are they related to each other?

The different levels of complexity

You can describe a human being at different levels. A molecule such as serotonin is at one such level. At this level you can find other molecules such as hormones (oxytocin), lipids, proteins, enzymes. One level higher are the cells, such as brain cells and liver cells. We are talking about a level higher, but this can be incorrectly seen as implying a value judgement; higher is better. In this context higher only refers to a more composite combination, which makes for a more complex level. Cells are made up of molecules and are therefore at a higher level. One level higher one finds the body organs, such as the brain and liver. And going one level higher brings us to the complete organism, the man or animal as a whole. Yet a level higher is the social group of which the human is a unit.

Things and processes

You may wonder why serotonin has been mentioned in the previous paragraph, but depression has not. That is because depression is not a part of the physical world. If you are depressed, then it is about processes, not over things which you can see or touch. Does this process take place at the level of molecules – serotonin – or the whole organism? We say you are depressed because your person as a whole has and shows problems. You undertake little, find it very difficult to get started, sleep badly, feel that life is barely worth living. You would like to know how this came about. Perhaps because of what you have experienced: divorce, or the death of a loved one? Perhaps your thyroid is not functioning properly and that is the cause? You take an antidepressant that regulates serotonin, and your depression vanishes as snow before the sun. Might your depression have been caused by too little serotonin? Depression is common in your family: could it be be because of your genes? With each of these four questions we seek the cause at a different level of the organism: the social level, the level of organs, the molecular level or the level of the genes.

The quest for the cause is in vain

This quest for the cause is however based on an incorrect premise. It suggests that the one level has no connection with the other. This is not so. Each higher level is composed of the elements of the lower level. The functioning of that higher level is therefore dependent on the qualities of the components of the level below. Your brains can only work well if your neurons and serotonin do their job. And your serotonin only works well if your genes coordinate well. This might create the impression that the lowest level is more important than the highest level, because a higher level is composed of lower levels. But even that is an unjustified conclusion, because the organism is a complex dynamic system. In such a system the characteristics of the higher level do not correspond on a one to one basis with those of the lower level components¹See Man thinks ahead, Nature succeeds through hindsight . Which means that if your serotonin is not working quite as it should, then your brain may not function quite as well as it should, but this is not always the case. Another characteristic of such a system is that the higher level also influences the qualities and function of the lower levels of which it is composed. This means that if you are recently divorced, your serotonin may not function quite as well as before, but this is not always the case. Thus, although the different levels are interconnected and influence each other, they do not react to changes within the system in a predictable, standard or invariable manner.

You cannot assign a single cause to it

What does this mean for depression? Abnormalities in serotonin have been established in people with depression. But you can not say that the depression arises because of this abnormality, because this abnormality is also found in people who are not depressed. You feel depressed after a divorce, but you can not say that the depression arises because of it, because plenty of people do not develop a depression after their divorce. And so you also can not say that genes or other components are the cause, although they do play a role.
There is still a good reason to make a distinction between all those levels if someone is depressed. I know somebody who has had a very severe depression, which also prevails in his family, as does suicide. When he experienced a depression, there was no obvious agent such as divorce or loss of a loved one. He recovered after using the right medication for him. Whenever he tries to stop the medication the depression follows quite soon after. I also know someone who, when he became depressed, experienced no relief with any sort of antidepressant, but gradually recovered with psychotherapy. Here it was a question of processing past traumas. Therefore it can be relevant to consider the different levels when dealing with your depression. But it is pointless to think that this gives you an insight into the cause. The only thing you know is at which level you can best direct your therapy.
Thus: debates around the question of whether a serotonin deficit or experiences such as a divorce or personal loss are the cause of a depression are non-starters as far as I am concerned. And insisting that everyone with a depression should be treated with psychotherapy, or with antidepressants, is a wrongful one-sided representation.

References

Gallagher S (2018): The Extended Mind: State of the Question. The Southern Journal of Philosophy 56:421–447.

Passingham R (2016): Cognitive neuroscience : a very short introduction. Oxford University Press. ISBN 978–0–19–108954–1, Ch. 1.

Craver CF (2015) Levels. In Open MIND Anthology, Open MIND Group. Metzinger T, Windt JM (Eds). ISBN: 978-3-95857-102-0. p. 689-740.

Smit H, Hacker PMS (2014): Seven Misconceptions About the Mereological Fallacy: A Compilation for the Perplexed. Erkenn 79:1077–1097.

Juarrero A (2002): Dynamics in Action: Intentional Behavior as a Complex System. A Bradford Book. ISBN 978-0-262-60047-7

van Praag HM, de Haan S (1979): Central serotonin metabolism and frequency of depression. Psychiatry Res 1:219–224.