Research by psychologist Benjamin Converse at the University of Virginia finds that human beings tend believe in a kind of karma, namely our western skew on the spiritual axiom that good deeds result being treated well by fate; we believe we can influence uncontrollable outcomes by performing good deeds, with the often underlying expectation that the universe will pay us back in kind. Confronted with bad news, we may think "If I can get through this, I'll be a better person from hereon."
Karma is thus a kind of reciprocity: I'll buy this round, you'll buy the next, however the deal is made with the universe itself, rather than specific individuals. It's an attempt to steer life towards expected and advantageous directions. We hope our acts of kindness to pave the way for journeys through life that are safe and not too challenging; we hope our kind words inoculate us from pain and discomfort; alas, life doesn't comply with these demands.
Yet, as the Buddha taught in the first noble truth, aging, sickness and death are in store no matter how we behave, and so to will we experience a wide array of setbacks, separations and frustrations, often on a daily basis. While skillful actions can sometimes bring about positive external situations; just as often our generosity is not rewarded with validation or financial rewards. Meanwhile, truly vile and harmful individuals often achieve notable successes in the world; Dick Chaney and Henry Kissingers, for example, are very rich, criminal assholes (which is an insult to anuses, frankly).
And so when our best efforts result in lemons we may react by losing conviction in our spiritual paths. We turn nihilistic, feeling duped by spirituality, as if we've purchased a product that doesn't act as advertised. "Why did I bother helping so and so when they now don't return my calls?" is an all too common refrain.
The Buddha's teaching on karma, however, is not a promise of pleasure, acclaim or financial rewards for our skillful deeds. It's not even a promise of a good rebirth, or even that rebirth actually happens. Note the Buddha's great teaching to a people known as the Kalamas:
"What if there is no life after death, if our actions, both right and wrong, have no external benefits? Still, here in our present lives, if we act without harmfulness or ill will, we will live in a state that is easeful, serene."
—kalamas sutta, Anguttara Nikaya, 3.65
In other words, if we act from generosity, good will, gratitude, we'll experience whatever life brings us with greater peace of mind; if we act aggressively, our psyches become agitated as result: no matter how wealthy we become, we wont truly enjoy it. Selfish and harmful acts result in feelings of lack of trust and connectedness with others, which is truly a dire state to find oneself in. I don't know how much misery Dick Chaney or Henry Kissinger experience, but I'm sure their internal realms would feel like hell to me.
So karma is primarily a psychological teaching, it means taking responsibility for all the decisions we make, becoming accountable to both the harm or ease to others than can result, understanding that its our emotional states, not external conditions, that can be influenced. It's one of the great teachings in spiritual life: karma doesn't change our universe as much as it changes how we interpret and experience life. And how we experience life determines our happiness and peace of mind. So, how would you like to proceed?