How Did It Start?
Stress may be physical, like a burn, or mental, like anxiety. In either case, stress causes the release of molecules called cytokines that may promote inflammation (or dive into this long account). Inflammation helps us to fight off infection, but in the brain it may lead tomental illness. So now we have so-called “cytokine hypotheses” to explain both schizophrenia and depression. If inflammation triggers mental illness, we have established links (more here) from our genes to body chemistry to inflammation to schizophrenia anddepression.
Our responses to stress are influenced by our genes, but a genetic influence does not mean that there is a single gene for schizophrenia or depression or addiction. Nevertheless, genes are known to alter our sensitivity to environmental influences. In turn, the environment can sometimes turn genes on and off. A complex set of interactions may result in a disorder like schizophrenia. This leads to a question.
Did psychological disorders arise by evolution? Did addiction evolve**? Was depression once an adaptive trait? Or schizophrenia? What does it mean if the mutations that led to mental illnesses occurred before humans existed?
(Your answer will depend on several considerations. First, the causes of behavioral disorders can’t be observed. That’s because the disorders are made up. They are not imaginary, but they are diagnostic categories—psychological constructs—rather than defective “things”. People suffer real pain but may not fit into boxes; the boundaries between one disorder and another reflect clinical opinion about symptoms alone, not causes. Second, some clinicians argue that mental illness cannot be reduced to brain diseases. Serious researchers may consider addiction a choice and not a disease***.
*This may be changing.
**The evolution of physical disorders is accepted widely and taught in high schools. Mental disorders are trickier and more interesting.
***Simple answers may make you impatient. If you look for answers in the brain you will have plenty of company. Addiction may not be a pursuit of pleasure or an avoidance of pain in the form of negative reinforcement so much as a compulsion. It may be not a liking but a wanting. NIDA Director Nora Volkow and psychologists Robinson and Berridge have suggested that drug stimuli exert such a powerful influence over the behavior of the addict that he or she wants nothing more than the drug. This is called the incentive-salience model. Here’s a quick summary of the view from Volkow and a longer one from Berridge.
The dopaminergic system of reward may be more of a system to focus attention on stimuli that predict reward. An even newer line of investigation is looking at how addicts make decisions, hypothesizing that the crucial problem may be dysfunctional ways of making choices. We should not call addiction a moral failure or even a disease, perhaps. It’s hard to deny that it’s a disorder.
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