In life , some of our decisions and actions are well thought out . They involve research , examination , and time for thought . And other decisions and actions are not well thought out . We make them quickly , without thinking , almost like a pure physical reaction . We can call these decisions , actions or reactions knee-jerk . This term comes from a medical process . Meriam Webster defines the noun "knee jerk " as “an involuntary forward kick produced by a light blow on the tendon below the kneecap .” It is an involuntary action . The words “involuntary ” and “automatic ” are important when understanding the term “knee jerk ” and when to use it . We often use “knee jerk ” as an adjective . Like hitting your kneecap to move your leg , knee-jerk reactions or statements also seem like an involuntary reflex . "Knee jerk " also describes predictable behavior . Again , hit the knee , the leg will kick every time . So "knee jerk " describes someone who always reacts quickly and without enough thought . For example , let's say you are on a team with someone who never faces responsibility for their mistakes in the game . If brought to account , the player always blames the dropped ball on other teammates . You can say that player is having a knee-jerk reaction to criticism . It's like a reflex they can't control . We often use knee-jerk in a criticizing or negative way . Now , a gut reaction also happens without thinking . But it is a bit different than a knee-jerk response . A gut reaction is a feeling . When we have a gut reaction , our instincts are telling us that something may not be right . Gut reactions are instinctual warnings . So , we also use this expression a bit differently than "knee jerk ." We usually talk about our own gut reactions or impulses , not others . Here is an example : One day , I was hiking on a mountain in California . I had hiked for about two hours and was near the top . The view , I was told , was truly beautiful . I couldn't wait to see it ! Then , suddenly , I had a strange feeling . Something just didn't feel right . I can't explain why , but my gut instinct was to turn around and head back down the mountain . I later found out that a dangerous bear had been sighted in the same area that I had fled . I am so thankful I listened to my gut instinct .