Why do we get goosebumps?

Because a tiny muscle at the base of each hair contracts and pulls the hair upright, tugging the surrounding skin into a bump. In humans it is a leftover reflex that no longer does much, but the machinery is intact and still fires.

Each hair follicle has an arrector pili muscle attached to it. It is controlled by the sympathetic nervous system — the same branch that raises your heart rate and diverts blood flow — and it is not under voluntary control. When it contracts, the follicle tilts, the hair stands up and the skin puckers.

In furred mammals this is genuinely useful. Raised fur traps a thicker layer of still air against the skin, improving insulation in the cold. It also makes the animal look larger, which is why a threatened cat's tail bristles. Humans have kept the reflex but lost the coat, so the insulating benefit is negligible and the intimidating effect is nil.

The two triggers reflect those two original functions. Cold produces goosebumps as part of a broader response that also includes shivering and narrowing of surface blood vessels. Strong emotion — fear, awe, a particular passage of music — produces them through the same sympathetic pathway, mediated by adrenaline. That is why the feeling accompanies both a cold draught and a moving piece of music, despite having nothing else in common.

The response is involuntary and brief, subsiding as soon as the trigger passes or the body warms.

Persistent, widespread goosebump-like bumps that do not come and go are usually something else — most often keratosis pilaris, a common and harmless blockage of hair follicles that has a different cause entirely.