Albert Demaret was also referring to these chaotic and bloody periods when he spoke about psychopaths: "In times of peace, we lock them up; in times of war, we rely on them and cover them with medals." Indeed, the soldiers who stormed the beaches of Normandy in 1944 or more recently invaded Bin Laden's stronghold were better off with psychopathic tendencies and a dose of paranoia rather than feelings of kindness.

According to Jérôme Englebert, slightly psychopathic tendencies may actually confer an individual advantage in the business world, or the workplace in general. He believes that certain morally dubious aspects of our society actually encourage behaviours whereby subordinates are regarded as objects, which allows society to evolve in a direction based purely on profit motive.

But let's go back to the manic-depressive syndrome. What is the adaptive advantage of the profound depression that is the other facet of the illness? Several hypotheses have been put forward. "One of them ascribed a protective function to the ‘bottom of the wave’. This brings to mind animals that hibernate, who hunker down during the winter season to emerge full of vitality in the spring," says Valérie Follet, a clinical psychologist who worked with Jérôme Englebert on the new edition of Demaret's book. Another hypothesis, put forward mainly by John Price, a British psychiatrist from Oxford, suggests that depression is adaptive because it allows the individual to withdraw from "social combat" when they believe they have very little chance of winning, and would be better off staying away when the social cost is too high.

The broken wing

Let us now turn our attention to the plover, a wading bird that builds its nest on the ground. When a predator or human approaches, the bird pretends to have a broken wing rather than flying off. When the danger comes even closer, the plover moves just several meters away. The predator believes they have an easy target: a bird that cannot fly well. But every time the predator approaches, the plover keeps pretending to be disabled, and flies off a bit farther. Studies have shown that the bird may even lead predators several kilometres away from the starting point.

What can explain this behaviour that is well known to ethologists and ornithologists? The consequence of this behaviour is instructive: by following the plover, the predator is drawn farther and farther away from the nest and the bird's young. "In functional terms, this behaviour is extraordinary," says Jérôme Englebert. "When the bird finally flies off and disappears, it has ensured its survival and that of its young." He adds, "some authors see this behaviour as proof that the animal shows intent. But I don't share that opinion. Even though the behaviour has a very obvious function, the question of voluntary intent remains open."