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So, testosterone fluctuates daily and seasonally and declines after the prime years. I get that and it seems to be well researched (we've known that testosterone fluctuates and declines over time for many years). Now we would need to establish a causal (or at least correlative) relationship between seasonal variation in testosterone levels and appearance of depressive symptoms.
Depression (conservatively defined) isn't a new phenomenon, but it seems to becoming more common around the world now than it was 50 or 75 years ago. The rate of mental illness (requiring intervention) used to be posted at 10% -- this about 50 years ago. Over time it has increased to about 20% of the population. IF 1 out of 5 people are experiencing diagnosable mental illness in various countries, then something pervasive (and unnatural) is going on. What could that be?
Just an off the cuff guess would be the on-going turmoil that industrialization and aggressively managed economic policy causes in both developed and developing countries. Capitalism, more or less, would be another term for it. Capitalist (or for that matter non-capitalist) industrialization subverts individual, family, and community life to the needs of hungry enterprises. People can cope with these changes for a while (a period of years) but eventually the cost of adaptation begins to erode resilience.
Take as one small example the opioid epidemic: A very strong opioid drug was heavily marketed to physicians and was presented as not being an unusual addiction hazard. Stupid. no opiate is non-addicting. Addiction is just a feature of the way opioids work. Doctors should know that, but representatives of the Purdue Pharmaceutical Company, LP presented it as a safe drug, and doctors tend to rely on dug companies for information. Plus many doctors, being at least as venal as everybody else, enjoy the perquisites they get for generously prescribing whatever is on offer. The Sackler family which owns Purdue Pharma LP are the beneficiaries of the boom in oxycontin sales and addiction. They also make hydromorphone, oxycodone, fentanyl, codeine, and hydrocodone, MS Contin, Oxycontin, and Ryzolt (time release Tramadol, an opiate). They also make iodine-based surgical disinfectant washes, laxatives, and -- humanitarian of the year award -- gluten-free stool softeners.
I don't have anything against opiates: When used with caution they are effective for relief of moderate to severe pain. What opiates are not good for is long-term use, where addiction is practically guaranteed. Opiate addiction is really not good for the body, especially when the drug of choice is not readily available. If one could buy opiates over the counter, addiction would still be a bad thing but it might result in less socially destructive dysfunctional behavior practically required to maintain a supply.
Anyway, the Sackler family drove the sales of its narcotic products to maximize profits for themselves with enormously negative consequences for millions of people, directly and indirectly. They are an egregious example, but not fundamentally different than what most wealthy corporate owners are willing to do. The Koch brothers are another egregious example, but so are the owners of Shell, Exxon, Rio Tinto, Amazon, Facebook, et al.
Poverty, addiction, social disruption, over-work, naked exploitation, zero free amenities (like parks, clean swimming beaches, etc.), no time and place for good sex, good food, nice music (however defined--Mozart for me, rap for you)--add very abrasive carborundum grit to the grind of already unsatisfactory life and wear people down to the point they can no longer cope effectively.