The term “crab mentality” is used to describe a kind of selfish, short-sighted thinking which runs along the lines of “if I can't have it, neither can you.” This term is especially widely used among Filipinos, who use it specifically to refer to people who pull other people down, denigrating them rather than letting them get ahead or pursue their dreams. As a general rule, an accusation of having a crab mentality is a poor reflection on someone's personality.
This concept references an interesting phenomenon which occurs in buckets of crabs. If one crab attempts to escape from a bucket of live crabs, the other crabs will pull it back down, rather than allowing it to get free. Sometimes, the crabs seem almost malicious, waiting until the crab has almost escaped before yanking it back into the pot. All of the crabs are undoubtedly aware of the fact that their fate is probably not going to be very pleasurable, so people are led to wonder why they pull each other back into the bucket, instead of congratulating the clever escape artist.
When someone has a crab mentality, it means that they are unwilling to allow someone to get out of a situation, or to get ahead. In a classic example of crab mentality, people who are attempting to get out of bad life situations often find themselves foiled by friends and family members who keep sucking them back in. For example, a Latina immigrant in the United States who decides to pursue a college education in the hopes of securing a good career may find herself discouraged by family members who do not approve of education, or fear that she will become distanced from her family after going to college.
The crab mentality can strike at all levels of life. Some charitable organizations are sometimes accused of a crab mentality, with members of the organization failing to think ahead or refusing to support certain initiatives in a way which ultimately drags the whole charity down. In office environments, the crab mentality can be particularly devastating, as coworkers snipe at each other to bring each other down, rather than congratulating someone who earns a promotion.
Filipino Culture
The crab mentality is a reflection of the famous saying “we all like to see our friends get ahead, but not too far ahead.” Learning to recognize the crab mentality in yourself is a very good idea, especially if you work or live in a highly competitive environment.
Source: http://www.wisegeek.com/what-is-the-crab-mentality.htm
