kneejerk

Kneejerk is a term used to describe an automatic, immediate reaction to a stimulus, characterizing a response that is largely unconsidered or reflexive. In everyday language it can function as both an adjective and a noun, as in "a knee-jerk reaction."

Originates from the patellar tendon reflex, commonly tested with a reflex hammer. When tapped, the quadriceps muscle contracts, causing the leg to kick forward—a rapid, involuntary response. The term's figurative use emerged in English to describe similar quick, reflex-like judgments or actions that do not involve deliberate thought.

In psychology and social discourse, knee-jerk responses are contrasted with deliberated reasoning, reflective decision-making, or carefully weighed arguments. The phrase is often applied to politics, media, and opinions to signal perceived impulsiveness or bias; critics may call for more evidence or reflection before forming a stance.

Notes: The knee-jerk reflex is a monosynaptic stretch reflex mediated by the spinal cord with the patellar tendon strike activating muscle spindles in the quadriceps. The sensory input travels to the spinal cord and directly activates motor neurons that cause contraction, with inhibitory input to antagonists allowing extension.