Isn’t it predicated on the notion of heredity and influence on sexual fitness? The assumption here being, somehow, there’s a genetic markup which positively or negatively influences propensity for depression, and that that has a positive / negative effect on procreation (either directly or through survival)? If the original effect of the genetic trait was different (as someone else mentioned) and it had a positive effect on fitness (survival), then that’s still a valid order of cause and effect analysis , and not assumption of intelligent design.
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