What Does Knee-Jerk Reaction Mean?

Knee-Jerk Reaction Meaning

Definition: A response that is automatic, rather than carefully thought out.

Origin of Knee-Jerk Reaction

This expression comes from the body’s natural reflexes. If someone hits a knee in the right place, the leg will kick outwards. This happens quickly, because the nervous system does not send the signal all the way to the brain and back. Rather, it is an automatic reaction that moves from the knee to the spine and back again, allowing it to happen speedily.

The “knee-jerk” was recorded in Sir Michael Foster’s 1877 Text-book of Physiology:

  • “Striking the tendon below the patella gives rise to a sudden extension of the leg, known as the knee-jerk.”

People have been using this expression figuratively since the early 1900s. In its figurative sense, a knee-jerk reaction refers to an emotional response that one does not think about before reacting.

For example, if someone says something rude about you in public, your knee-jerk reaction might be to call that person a nasty name.

In most cases, a knee-jerk reaction is not considered a good thing. Since it is an automatic response, little thought is ever given to knee-jerk reactions. For this reason, they, generally, have a negative connotation.

Examples of Knee-Jerk Reaction

knee jerk responseIn the first example, two employees are talking together on their lunch break.

Marcus: It’s nice weather out there today.

Patsy: Yeah. I’m glad winter is finally over. Wait, you aren’t going to eat those cookies, are you?

Marcus: Why shouldn’t I? Do you think I’m too overweight? I think it’s very rude of you to assume you know better than I do about what I can and cannot eat. Not that it’s any of your business, but I’m on a strict diet and I’ve been exercising two hours every day. These cookies are the only unhealthy snack I’ve had in days!

Patsy: Whoa, whoa, whoa! That’s not what I meant at all. I just meant that those cookies expired a few months ago. You might get sick if you eat them.

Marcus: Oh, I’m sorry! I just had a knee-jerk reaction because I’m really sensitive about my weight. I always assume people are judging me for it.

Patsy: I would never do that. Next time don’t assume that.

define knee jerkIn the second example, a brother is trying to prank his sister.

Samson: Boo!

Angela: Ahhhh!

Samson: Hey! You almost punched me in the face. I’m lucky I was far enough away from you that you didn’t reach me.

Angela: Well, if you don’t want to get punched you shouldn’t scare me. Defending myself is just a knee-jerk reaction. I thought I was in danger.

More Examples

This excerpt is about a man complaining about a bill. The bill would allow motorists to use their cars to legally hit protesters. The man believes that the bill makers did not think carefully about the problems this would cause.

  • Goldtooth called the bill and others like it “a knee-jerk reaction that carries the sour taste of racism.” –LA Times

This excerpt is about how an environmental protection law did not happen spontaneously. Rather, scientists and lawmakers carefully considered the reasons behind it before taking any action.

  • In other words, there was nothing “immediate” about the EPA’s action. Nor was it a “ban,” if one recognizes that manufacturing in the United States and marketing and sales overseas were wide open. If Offit is implying that the EPA banned DDT as some sort of knee-jerk reaction to public pressure, that’s not supported by the record. –LA Times


The phrase knee-jerk reaction means an immediate reflex that doesn’t involve thinking about the response beforehand.