Before we even look at the science of Karma we should consider religion. In Buddhism and Hinduism; Karma is viewed as the result of a person’s actions in this (and any previous) existence, as well as the actions themselves, that decide what will happen to that person in their future existence.
Karma is not unique to eastern religions. In Christianity there is also a sense of Karmic destiny in the judgement upon death of one’s life’s actions and their impact. In the Jewish faith Karma is expressed as Middah k’neged Middah (translated as Measure for Measure). The more popular take of it is found in “what goes round, comes round”.
There is such pervasive belief at a deep, intuitive level in Karma that even the ancient Greeks who believed that we’re all bound to a predetermined fate that just needs to be manifested, also managed to believe in a type of Karma where the actions of a person eventually were visited upon them.
Nothing happens without a reason. It is ironic that I am using this statement to explain Karma as Karma itself is popularly, and somewhat erroneously, understood as a cycle of cause and effect. We are each products of our environment. That environment supplies stimuli which our brains interpret into signals that give rise to emotions, feelings and ideas.