Is othello's tragic flaw -- the quality that leads to his downfall -- jealousy or something else, perhaps credulity?

Othello is a man with a noble heart. However, his is a very insecure man.

His main flaw is his insecurity. Being insecure made him vulnerable to Iago's manipulations, he became jealous and gullible. He believed what Iago has been telling him even though it is all a lie. 

In the end, his insecurities resulted to his jealousy, driving him to kill his own wife and leading him to his tragic downfall.