After dinner last night, I lost my usual self-control and ate half a box of cookies. No wonder. My self-control had been under pressure all day. I righteously refused a muffin at breakfast, didn’t scream at my kid to get out the door although we were late, made a conscious decision not to run over a pedestrian crossing against the light, kept my fist from pounding on the table during a faculty meeting, and resisted the urge to throw an annoying student out of my office. But by 7 p.m., my self-control mechanism was worn out, and down those cookies went. The empty box would have been no surprise to Yale University psychologist Joshua Ackerman and colleagues who have discovered that self-control not only wears us down, even thinking about other people's self-control is too much to handle. In the latest issue of the journal Psychological Science, the researchers taunted subjects with the story of a waiter who was surrounded by gourmet food but not allowed a taste. Some of the subjects were encouraged to go beyond polite listening and actually imagine this poor waiter, to have real empathy with his situation. And then everybody was shown pictures of expensive stuff. Those who had put themselves in the shoes of the waiter, had suffered all that self-control as he had, wanted that stuff, no matter the price. In other words, just the thought of someone, anyone, depriving himself eventually makes greedy beasts of all of us. Apparently, it's human nature to be out of control. Imagine our early ancestors roaming the savannah looking for food. They might bring down a gazelle, but that meat was probably not enough for some of the group. As soon as they wiped their mouths, those lacking self-control were probably off again on the hunt because they could not deny themselves anything. Such an attitude was probably adaptive. It kept the group on the take, always looking, always wanting, always getting, and those who wanted more surely lived longer and passed on more genes that those who sat around the first gazelle and said, "We'll, I'm satisfied," not imagining they would be hungry again soon. The need for self-control must have come much later, and in other spheres than food. Group living, for example, takes great self-control; it takes a lot to live with people day after day and not kill them, and so those more reflective humans who could keep their anger in check probably did well once humans settled into communities. But that kind of self-control has become so painful in the modern world because there is so much to want, so much to tempt our restraint. We live in busy, complex communities surrounded by desirable goods and fun ideas, and so all day, every day, we hold back. And we see that most everyone else is holding back too. We are hit hard by both our own weary self-control as well as the exhausting empathy we apparently have for everyone else's self-control. It really is too much. It makes perfect sense that we sometimes lose it and eat half, or even a whole, box of cookies in one sitting.

Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link). Her Human Nature column appears each Friday on LiveScience.