Using extremely precise measuring techniques, the researchers were able to determine how long it took for the participants to shift to the new image. It took slightly longer to shift the gaze from a beautiful face than it did from an average face, and that held equally true for both female and male participants. The difference was about 100 milliseconds. (One millisecond is one-thousandth of a second.)
That held true, even though the face disappeared just as the other image appeared. If the face was beautiful, the participants continued to stare, ever so briefly, at the spot where the face had been — something scientists call the "episodic memory" of the face.
The researchers selected their photos after submitting them to a group of undergraduates who rated them according to physical beauty. That may sound a bit arbitrary, but Maner said deciding who is physically beautiful, and who is not, is not as romantic as it might seem.
"Although a lot of people say beauty is in the eyes of the beholder, there's a great deal of agreement across [professional] observers, in terms of who is attractive and who is not," Maner said.
"There's actually a large and intriguing body of literature on what specific features people find attractive. One of the big ones is facial symmetry — how one side of the face mirrors the other. Symmetry is a sign of good genes and overall health."
"Attractiveness in men includes markers in testosterone, like a square jaw. In women, it's facial features associated with youth — a high forehead and big eyes. So, there are very objective qualities that we use to judge someone's personal attractiveness, even though we don't really realize we are doing it."
But, in the case of jealousy, in which participants fixated on images of beautiful faces from their own gender, were men and women the same? Both genders fixated on members of their own sex in apparent recognition of a formidable rival, but most men would be unlikely to admit that.
"It's funny, men are often less willing to call other men attractive," Maner said. "Women are very willing to say that other women are attractive. I'm not sure why. Maybe latent homophobia."
Maner cautions against using this study to soothe the troubled waters of a relationship soured by one partner spending too much time sizing up someone else.
"The fact is that, although the immediate response may be involuntary, there's certainly all sorts of chances to use self-control, to use one's own will to pull attention away."
In other words, the grace period is only 100 milliseconds.
Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.