People in the west seem to have very little idea what "karma" actually is. They tend to think of it as some kind of mysterious cosmic force that ensures good things happen to people who do good things and bad things happen to people who do bad things. If you actually look at Hinduism and Buddhism, you find a very different story however. Karma is actually a name for causation—the idea that causes have effects, and people are responsible for the effects of the things they cause. In this sense I don't think anybody can reasonably doubt the existence of karma.