Everything good in life is illegal, immoral or fattening—or so the saying goes. A few centuries ago religious authorities sought to codify that sentiment into a handy list, which we know today as the seven deadly sins. In this special issue devoted to them, we explore how desires take shape and influence our thoughts, alongside the scientific insights that can help us meet our goals.

We often think of temptations as the ruin of diets, oaths and ambitions, yet their pull is a natural part of life. They can even be meaningful. As psychologists Jan Crusius and Thomas Mussweiler suggest in “Envy: The Feeling Can Help Us Even When It Hurts,” envy may alert us that we face a disadvantage, thus motivating us to take action.

Pride, too, compels us to try harder so as to feel good about ourselves and to secure high status. In the form of arrogance, pride elevates social standing while alienating others. Yet it can also be a positive force. Psychologist Jessica L. Tracy explores the dynamics of hubris and self-esteem in “Pride: It Brings Out the Best--and Worst--in Humans.”

Sometimes factors beyond our awareness can cause trouble. The makeup of a meal, for example, can nudge us toward gluttony. To temper the urge to overindulge, think carefully about the flavor profile of your next dinner, selecting spicy foods over bland ones because we reach satiety sooner when tastes are piquant. Click here for “Gluttony: Are We Addicted to Eating?,” by contributing editor Karen Schrock Simring.

At other times our actions can baffle us, such as when we lash out at the people we love. To quell rage toward a romantic partner, psychologists Eli J. Finkel and Caitlin W. Duffy find that writing about a conflict in the third person can reduce a couple's anger and distress. They describe why flare-ups occur and how to handle them in “Wrath: How Intimacy Can Breed Violence.”

These fascinating topics and others lurk in the mind's so-called dark side, and exploring it can leave us both wiser and stronger. Illegality may be fairly cut and dry, but immorality is not—and therein lies the fun.