Asked by: Sundus Ilott
Head-scratching is an example of what biologists call a ‘displacement activity’, where an animal that’s unable to choose between two conflicting options will opt for neither and instead perform some unrelated behaviour. For example, a bird that’s unsure whether to attack or flee from a rival might suddenly begin to peck at the ground.
Originally, these behaviours – which can also include preening and grooming – may have emerged because the soothing physical contact, or the familiarity of a routine behaviour, acts to calm a stressed animal.
But the behaviours have evolved into part of the non-verbal language that animals use to signal their emotional state. A 2017 study at Portsmouth University found that macaques scratch their heads when stressed, and this acts as a signal that makes other macaques less likely to attack or harass them.
So we may have inherited this social cue from our ancestors, perhaps as a way of warning people to leave us alone when we’re thinking hard.