Brain Involuntary
A pleasant tingling that starts on the scalp and moves down the neck and spine, set off by whispering, tapping, soft sounds, or gentle personal attention. Not everyone gets it. A large online community is built around it.
Unclear; estimates 20–70%
Muscle Involuntary
Small visible twitches in the eyelid, calf, thumb, or arm that come and go with no weakness. About 70% of healthy people get them. They are not a sign of nerve disease.
About 70% of healthy people
Ears Involuntary
A short, intense spinning feeling set off by turning over in bed or tipping your head back. Tiny crystals in the inner ear have drifted into the wrong place. It is the most common cause of vertigo and it is very treatable.
About 2.4% of people, lifetime
Nerves Involuntary
A short, sharp headache in the forehead a few seconds after cold food or drink touches the roof of the mouth. It passes in under a minute.
About 30–40% of people
Brain Involuntary
A brief feeling like a small electric shock inside the head, often set off by moving the eyes. It is most common when stopping or lowering an antidepressant. It is unpleasant but not dangerous.
15–50% of people stopping an antidepressant get withdrawal symptoms
Brain Involuntary
A sudden, brief thought of jumping when you stand somewhere high, or of swerving while driving, with no wish to do it. About half of people get it. It is the brain's safety system being misread, not a hidden wish.
43–60% of people
Each of these takes under a minute and needs nothing but your own body.
Eyes Perception
An image that stays in your vision after the thing you looked at is gone. Bright things leave a same-colour copy for a moment. Long stares leave a copy in opposite colours.
Everyone
Brain Trick
Using both hands equally well for every task. Fewer than 1% of people are truly ambidextrous. Most people who say they are ambidextrous are mixed-handed: right hand for some jobs, left for others.
Under 1%
Ears Variation
Some earlobes hang free and others join the head directly. Textbooks taught this as one gene, but a 2017 study found at least 49 genetic regions involved, and the two shapes blend into each other.
About 61% attached in one Korean sample
Mouth Variation
The uvula, the dangling flap at the back of the throat, is split into two lobes, in about 1 to 2% of people. On its own it is harmless, but in a baby it can be a clue to a hidden cleft of the palate.
1–2% of people
Eyes Perception
Look at a small red light in a dark room and two faint blue arcs curve away from it towards your blind spot. You are seeing the nerve fibres of your own retina light up.
94% of healthy eyes in one clinic test
Eyes Perception
Tiny bright dots that dart along wiggly paths when you look at a clear blue sky. They are your own white blood cells moving through the capillaries of the retina.
Everyone
Things that alarm people and turn out to be harmless. Each entry says when a doctor is still a good idea.
Bones Variation
Small extra bones, most often in the foot, that form when a growth centre never fuses with its main bone. They are harmless but can be mistaken for fractures on an X-ray.
Up to 1 in 5 feet
Brain Perception
Objects, body parts, or the whole room suddenly look far too big, far too small, or far away. It mostly happens to children, often during a fever or infection, and it passes on its own. In adults it is usually linked to migraine.
6–7% of teenagers (size distortion)
Bones Variation
An extra rib above the normal top rib, growing from the lowest bone in the neck. About 1 in 100 people have one, and most never know.
About 1% of people
Brain Perception
People who lose a lot of their sight often start to see things that are not there: patterns, faces, small people in costume, whole landscapes. They know the images are not real. It is caused by the eyes, not the mind.
10–40% of older people with serious sight loss
Brain Involuntary
The world suddenly seems flat, distant, or like a film (derealization), or you feel like you are watching yourself from outside (depersonalization). Most people have a brief episode at some point, usually under stress.
50–75% of people, at least once
Muscle Variation
A widening of the gap between the two halves of the six-pack muscle, seen in nearly every pregnancy and in some men. A bulge down the midline when you sit up is the usual sign.
Almost all pregnancies; about 1 in 3 women a year after birth