The “Iliad” may be a giant of Western literature, yet its plot hinges on a human impulse normally thought petty: spite.

Achilles holds a festering grudge against Agamemnon (“He cheated me, wronged me … He can go to hell…”) turning down gifts, homage, even the return of his stolen consort Briseis just to prolong the king’s suffering.

Now, after decades of focusing on such staples of bad behavior as aggressiveness, selfishness, narcissism and greed, scientists have turned their attention to the subtler and often unsettling theme of spite — the urge to punish, hurt, humiliate or harass another, even when one gains no obvious benefit and may well pay a cost.

Psychologists are exploring spitefulness in its customary role as a negative trait, a lapse that should be embarrassing but is often sublimated as righteousness, as when you take your own sour time pulling out of a parking space because you notice another car is waiting for it and you’ll show that vulture who’s boss here, even though you’re wasting your own time, too.