Introduction
Depression seems, in appearance at least, to be caused only by the environment. Indeed, the most acknowledged causes of depression are a difficult childhood as a long run cause and traumatic events as starting causes. Thus, depression would only be a specific reaction to a particular environment. The fact that we acknowledge nurture as the only determining factor of depression as a human behaviour as great consequences: depression is not really considered as a disease, but more as a general feeling (feeling depressed), or the belief that anyone is, in theory, equally vulnerable in front of depression.
However, some data are hardly explainable with an only nurtural model of causes. For instance, an only nurtural etiological model of depression can not explain the fact that the heredity rate of depression is higher for women than for men. Therefore, we ought to introduce and argue for Nature especially genes as a cause of depression. In this paper, we will introduce, analyse and evaluate the importance of Nature as a cause of depression, thus denying Nurture the status of the sole and most important factor in determining depression.
In this respect, we will provide the reader with a twin experiment that proves the influence of genes in depression. This twin experiment will introduce Nature as a cause of depression. Then, in a deepened analysis of symptoms and chemistry of depression, we will explain how genes contribute to depression. Thus, the role of Nature in depression will be précised and analysed. At last, in confronting natural causes and nurtural causes related to depression, we will propose a model of causation that includes both Nature and Nurture but that shows the Nature’s path in causes of depression, worthily introducing the ideas of genetic vulnerability and of a nurtural expression of Nature. To conclude, we will summarise what have been argued in this paper, and develop the contribution of the work achieved to the eternal debate Nature VS Nurture.
Twin studies: the introduction of Nature in Depression causes
In this first part, we will introduce Nature as a cause of depression. Therefore, we need an experimental setup that could test and evaluate separately the role Nature and Nurture plays in depression. In this respect twin studies are the perfect experiment to evaluate the part Nature and Nurture take in depression starting process. Indeed, in two twins, either if they are monozygotic twins (who has exactly the same genetic makeup), or dizygotic twins (who shares half their genes), who have shared an environment (raised in the same family), or who have not shared any environment (raised separately), the analysis and the comparison of similarity between two siblings lead to an evaluation of the role Nature and Nurture plays in the development of depression.
Thus, we need three groups. The group 1 is monozygotic twins that have shared the same environment. The group 2 is monozygotic twins that have not shared the same environment. The group 3 is dizygotic twins that have shared the environment. In comparing the similarity in a pair of twin from one group to the similarity in a pair of twins of another group, we can state whether Nature or Nurture play a role and eventually evaluate which of them is more important.
According to the ISMHO (the International Society for Mental Health Online), the similarity rate concerning depression is 76% for the group 1; 67 % for the group 2; 19% for the group 3[1]. The results are presented in the table below.
|
|
Monozygotic twins |
Dizygotic twins |
|
Same environment |
Group 1 : 76% |
Group 3 : 19% |
|
Different environment |
Group 2 : 67% |
|
In analysing and comparing the similarity rates between group 1 and group 3, we test the influence of Nature through genes in the causes of depression. Indeed, monozygotic twins share all their genes, whereas dizygotic twins share only half their genes, and twin pairs of Group 1 and 3 have shared the same (familial) environment. Firstly, if the similarity rate is the same for both groups, it means that genes do not play any role in depression. Secondly, if the similarity rate is greater for group 1 than for group 3, it means that Nature plays a role in depression. Moreover, the greater the difference is between Group 1 and Group 3, the greater the role of Nature is, compared to the role of Nurture. Here, the similarity (in depression) rate for Group 1 (monozygotic twins + same environment) is 76%, whereas the similarity rate for group 3 (dizygotic twins + same environment) is 19%. We first see that these rates are different, and that the similarity rate is far greater for group 1 than for group 3. Therefore, it means that Nature (or genes) plays a great role in depression. We also see that the difference in similarity rates (between group 1 and 3) is 57.
In analysing and comparing the similarity rates between group 1 and group 2, we test the influence of nurture through the familial environment in the causes of depression. Indeed, we compare monozygotic twins that have been raised in the same environment to monozygotic twins that have not been raised in the same environment. Firstly, if the rate is the same, it means that nurture does not play any role in depression. Secondly, the greater the difference between rates of Group 1 and group 2 is, the greater nurture’s role is, compared to the role of nurture. Here, the similarity rate (in depression) for group 1 (monozygotic twins + same environment) is 76%, whereas the similarity rate for group 2 (monozygotic twins + different environment) is 67%. We first see that these rates are different, which means that nurture plays a role as well. But we also see that the difference between similarity rates (between group 1 and 2) is 8.
In comparing the differences between similarity rates of Group 1 and 3 (same genes), to the difference between similarity rates of group 1 and 2 (same environment), we see that the difference in similarity rates is much greater for group 1 and 3 than for group 1 and 2. It could mean that nature plays a greater role than nurture in determining depression.
This twin experiment shows that nature plays a role in depression, and that this role could be even greater than nurture’s role. But it also provides lot of additional information that can only be explained through a deepening of genetic and chemical mechanisms in depression determination. Two main facts are striking. The first fact is that the similarity rate between twins is greater for female twins than for male twins. The second fact that the similarity rates decrease with the precision of the definition of depression: the more symptoms you include in your definition, the less similar twins are[2]. These facts can hardly be explained by an only nurtural system of cause.
We have seen that Nature play a great role in determining depression. But now in deepening our knowledge about chemistry and genetics of depression, we will see in details the contribution of genes in depression.
Chemistry and Genetics of depression
In this second part, we will deepen our knowledge about chemistry and genetics of depression, in order to be able to explain the data we have obtained in the first part. In this respect, we will adopt an etiological model but only for natural causes. Therefore we will start with a quick overview of the symptoms of depression; then we will see that these symptoms correspond to a specific brain activity and especially chemically. At last, we will go to the genetic level, and explore how genes could be responsible for a vulnerability to depression.
First, we have to explain more precisely what depression is. Depression is characterized by numerous symptoms. These symptoms are mood disorder (overall sadness, instability in mood, excitation, or breakdown), sleep disorder (insomnia or hypersomnia), eager disorder (bulimia, or anorexia), sexual disorder… The presence and intensity of these disorders differ from one kind of depression to another and from one person to another.
On a physiological level, these disorders correspond on to a specific and complex chemical activity of the brain. This chemical activity consists in a deregulation of interneuronal communication. This communication is made via neurotransmitters and neuroreceptors. Depression is on a chemical level a deregulation of this communication between neurons. However, not all neurochemicals are involved in depression. Indeed, it seems that depression is a deregulation of specific neurochemicals that are responsible for mood, for sleep, for eager, for stress… Three main chemicals have been identified as greatly linked to mood: norepinephrine, dopamine, and serotonin. Here we will analyse only the case of serotonin, but the functioning is identical for all neurochemicals.
Serotonin is a neurochemical that produces a partial inhibition of the nervous system, thus having the function of calming, and giving the impression of contentment and satiety. A disorder in serotonin regulation is partly responsible of depressive behaviour: indeed, depression is especially characterized by difficulties in staying calm, and by the feeling of absence of contentment and satiety.
But what is important here are the mechanisms that cause these chemical deregulations. Is it nature? Is it Nurture? Both of them? We know that environment, for example traumatic events, causes a specific chemical activity that creates emotions, for example stress, anxiety or awareness. However, these environmental events create the chemical activity, not the chemical deregulation. It is important to understand that depression is not a set of normal emotions like stress, anxiety or sadness, but rather a deregulation of these emotions. Therefore, if the environment causes emotions but not the regulation of emotions, we have to suppose that this regulation of emotions has a genetic origin. Thus, we have to go to the genetic level, taking the example of serotonin, in order to show that genes are responsible for regulation or deregulation of the emotional/neurochemical system.
Serotonin is a type of neurochemical. In this category, there are serotonin neurotransporters and serotonin neuroreceptors. The gene that codes for serotonin neuro transporters exists in two versions: the short version and the long version of it. This fact, together with the fact that chromosomes works in pair allows one to consider three possible genotypes: either [long version – long version], or [long version – short version], or [short version – short version]. The phenotype is the expression of the genotype, that is to say that the physical characteristic that results from the genotype through genetic mechanisms. Here, the phenotypes are clear. According to a study led by Dr. Avshalom Caspi and Dr. Terrie Moffitt, 33% of the persons that have genotypes [short-short] or [short-long] lipped into depression facing traumatic events, whereas that number is only 17% for the persons that have genotype [long – long][3]. These results show that serotonin genes are partly responsible for the development of a deregulation of the emotional / neurochemical system.
One problem of genetics is to explain the variety of behaviour. Indeed, the reduced possibilities of genotypes as well as those of the phenotypes, offer a poor explanation of this variety. However we must not forget that behaviour is a complex activity, and therefore it involves lot of elements, and therefore a lot of genes. It is through the mechanism of genetic combination that a unique and complex behaviour is created. In the depression case, the number of 19 genetic regions has been proposed recently by Dr. George Zubenko[4]. It would then make a lot of varieties of symptoms and diversity in intensity and duration of these symptoms.
Moreover, with this genetic model we can explain why the heredity rate for women is greater than the heredity for men. Indeed, it seems to indicate that one of the genes responsible for a vulnerability to depression is on the chromosome X which only women have in pair.
We have seen that depression symptoms correspond to a deregulation of the neurochemical system determined by genetic origins. We have also seen that with genetic combination, the natural model is able to explain variety of depressive behaviour. But, some supporter of the nurture cause could still critic the natural model, since environmental elements such as a difficult childhood or traumatic events cause depression. Therefore, we need to precise the role of these nurtural causes and see how nature can meddle in nurture, express itself in nurture.
Etiological model of depression: Nature and Nature in Nurture
In this part of the paper, using the data we gathered so far, we will propose an etiological model, an overview of causation in depression, Thus, we will precise the role of Nature in causes of depression, and develop further the concept of genetic vulnerability. This etiological system will allow one to consider and to define the Nature’s path in causes of depression.
First, we ought to classify causes. The first distinction is to be made between causes of depression, and causes of vulnerability to depression. Indeed, genes are not directly responsible for depression otherwise depression would be an automatic expression of genes. Indeed, it is not the case since the statistics we used above clearly shows that genes code for a vulnerability to depression (especially the fact that monozygotic twins have not 100% concordance concerning depression). This concept of vulnerability is to be developed because it is indispensable in understanding the role nature can have in human behaviour. Indeed, if genes code for vulnerability to depression, or more generally for preferences, genes are somewhere, somehow responsible for human behaviour. Then, the environment would only have the role of developing and precising these genetic preferences. Therefore, concerning depression, some causes are responsible for vulnerability to depression, whereas other causes are starting causes of depression. For causes of vulnerability, we ought to consider genes and difficult childhood. The starting causes, causes that are directly responsible for depression, are mainly traumatic events.
The second distinction is of course the separation between Natural causes and nurtural causes. In the case of depression, the natural causes are as we have already seen, genes, but we ought to differentiate between one’s genes and one’s parent’s genes, one’s parent’s genes being responsible for one’s genes. The nurtural causes of depression can be numerous but we can group them in two elements: a difficult childhood (death of one the parents, one depressive parent…) and traumatic events (divorce, difficult job situation, illness…). In appearances, this distinction between Nature and Nurture may seem clear and simple. However, it is interesting to see how they combine each other and how natural causes may be found into Nurture.
The first nurtural expression of nature is through childhood. Indeed, the difficulty of childhood is often to be found in relationship between the child and the parents. We already know that one’s genes are totally determined through genetic mechanisms by one’s parents’ genes. But parents’ genes can also have a great influence on a person childhood. Indeed, the more vulnerable to depression one is, the more likely his parents are to be also vulnerable to depression. And one of the main causes of a difficult childhood is a depressed parent, or the suicide of one of the parents, or separation of the two parents (that can occur when one of the parent is depressed, or that can cause depression of at least one of the parents). Therefore, to say that Childhood is purely nurtural can hardly be strongly backed.
Secondly, Nature also draws its path in depression through circular causation. Indeed, depression causes a modification in cognition, in one’s perception of the environment. The fact that cognition is modified by depression is really important: it allows one to consider that traumatic events are not traumatic as such, but that they are perceived as traumatic, especially when one is already depressed. Circular causation thus plays a role of deepener of depression. But if nature is, through vulnerability, one of the causes of depression, therefore it also contributes to this particular cognition that made an event to be perceived as traumatic. Therefore, we ought to consider the path Nature draws in causes of depression. Even what was considered as pure nurtural causes (Difficult Childhood and traumatic events) can have a natural origin.
This etiological model is presented in the scheme below:
We see that depression requires vulnerability and traumatic events to be started. We also see that Nature, through the parent’s genes contribution to a difficult Childhood, or through circular causation, can be found in every cause of depression. We also can infer that the limit between a depressive person (a person who has a genetic vulnerability to depression) and a non-depressive person cannot be straightforwardly established. Some persons will not be vulnerable to depression and therefore have no symptoms of it, even if they live traumatic events, or had a difficult childhood. Others will be only slightly vulnerable to depression, and have no symptoms because the environment did not start it. Others will have only few symptoms whereas some will have the whole set. But in each way, nature draws a path, a reduction of the infinite environmental possibilities to a very few set of possibilities in behavioural response.
Conclusion: Depression, a natural characteristic of human behaviour
In introducing nature as a cause with a twin experiment, in precising the role played by genes in determining vulnerability to depression, and at last in proposing an etiological model of depression, this paper lead one to consider the great role played by Nature in depression. Indeed, Nature through genes not only plays a role in depression, but it plays a major role, determining through genetic mechanism a genetic vulnerability to depression, and resulting through genes combination into a lot of variety of depression. Moreover, as we have seen before, Nature also expresses itself through Nurture, either in determining partly the childhood you had, through your parents genes, or through the process of circular causation that deepen depression. In this respect, this paper contributes to the cause of Nature in the eternal Nature VS Nurture in showing that genes draws a behavioural path that even Nurture cannot change. Then nurture would only determine the temporality of behaviour, but always in that famous natural path.
[1] Source : International Society for Mental Health Online, allaboutdepression.com
Available at: http://www.allaboutdepression.com/cau_03.html#1
[2] Laura Jean Bierut, Major Depressive Disorder in a Community-based Twin Sample, Archives of General Psychiatry vol. 56 no. 6, June 1999
[3] The main results of this study are available at : http://www.mental-health-today.com/dep/departlcle.htm
[4] Results report available at : http://www.medicalnewstoday.com/medicalnews.php?newsid=3880

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