Wysiwyg, I've done a lot of reading on karma over many years. Nothing from Buddhist literature ever satisfied me; the explanations were all fear-based. I've been on forums and asked and questioned and argued with people, and just recently I came across this explanation from a guy on a 'yahoo answers' thread, of all things. It felt right. Hope you like it.
--------------------------------------------------------------------------------
"I was raised in the practice of raja yoga, including the concepts of karma.
For 11 years now, I have been taking weekly lessons in Tibetan Buddhism from one of the Dalai Lama's monks living in my city ... new students ask a LOT of question about karma, so the topic is well-covered.
I also have a degree in psychology.
No. Karma is not a milder version of belief in heaven/hell.
The belief in heaven or hell is that it is a judgment by a Higher Power. It contains the concepts of punishment or reward ... and both punishment and reward rely on a power outside ourselves that is capable of punishing or rewarding. Psychologically speaking, it is a concept of "external locus of control", which incidentally is associated with higher incidences of clinical depression.
The belief in karma is that it is the same as a neurochemical imprint made into our brain, an imprint made by every thought, action, speech and feeling of ours. And that however he have made ourselves to be by setting these imprints within us ... we will eventually draw to us those circumstances and people that MATCH our internal imprints. Certainly, if you believe you cannot do math, you are doomed to flunk math. And certainly if you are rude and angry, you do not draw laid-back pleasant people to you. Karma says that these imprints have no limitations on their effect on our life circumstances.
The belief in karma is a belief in self-creation and in taking self-responsibility for what we have created. Furthermore, it sets us with the task of being mindful of whatever we DO encourage within ourselves, through what we choose to say, do, think, and feel (our feelings are the reaction of our thoughts ... and the only reason it doesn't always feel that way is because we often function without being fully aware of ourselves ... which is why Buddhists meditate .. to develop mindful awareness).
Psychologically speaking, karma is a concept of "internal locus of control" ... which is regarded as being more conducive to mental health.
-------------------------------------------------------------------------------