Is Depression Worse for Humans?
A serious question that deserves a serious answer
Posted Apr 21, 2011
I see moods as a legacy of evolution that help to flexibly tune behavior to situational requirements. The capacity for moods is useful. When a situation is favorable, high moods lead to more efficient pursuit of rewards. Reward-seeking behavior is invigorated (eat grass while the sun shines). In an unfavorable situation, low moods focus attention on threats and obstacles and behavior is pulled back (hunker down until the blizzard ends). Mood reflects the availability of key resources in the environment both external (food, allies, potential mates) and internal (fatigue, hormone levels, adequacy of hydration), and ensures that an animal does not waste precious time and energy on fruitless, or even dangerous efforts (doing a mating dance when predators are lurking).
Moods are not uniquely human, but are seen broadly in mammals, who, like us, have a capacity for wakeful alertness and a capacity to feel pleasure and pain. So it makes good sense to talk about depressed dogs, depressed rats, or depressed cats. Departing from this premise, considerable research has tried to model human depression, as well as the effects of human antidepressants, in our fellow mammals.
That said, some have claimed that humans have special troubles with long-lasting serious depression. Often such observations amount to no more than a quick throwaway line. After all, there aren't comparative epidemiological studies of depression in different species, and there won't be any anytime soon. Still, the argument that depression cuts a deeper ravine in our species should be pursued in a sustained way. We see signs of an epidemic of serious depression in humans. In the West, nearly one in five adults will have serious depression over the lifetime, with remarkably elevated rates of depression in young people. I'm not aware of any similar trends in tigers or tree shrews. We are the only species whose members regularly commit suicide when depressed.
Some pathways into depression run straight through the environment. Make the environment nasty enough and all but the hardiest will show signs of depression. From human torture victims at Abu Ghraib to Harry Harlow's monkeys, this environmental route is shared across the mammals. So I don't think it can explain the epidemic of human depression.
So, are there human-specific pathways into depression that explain our species' special vulnerability?
While there are many possible hypotheses, I believe the most credible is that our uniquely human cognitive capabilities are rendering us more vulnerable to depression, even without a major environmental insult. This is a potentially compelling perspective because the depression epidemic is unfolding at a moment when objective conditions for survival and reproduction are relatively benign. Over the last 250 years, people live longer, hunger is rarer, living standards for the average person have dramatically increased. The wolf literally is not at the door. Yet more people are depressed.
We usually think of human uniqueness with feelings of pride-be it our advanced language, our ability to reflect upon the past and plan for the future, or our ability to reference a shared culture. So I must leave you with the question that I am now pondering: How can a species capable of harnessing fire and putting a man on the moon be so often impotent in the face of low mood?