So, you're at a party with your favorite squeeze and you notice that a stunning specimen of the opposite sex has just sauntered into the room. How long does it take you to lapse into a blithering state of semiconsciousness, thinking the new arrival is the only other person in the room?
Less than half a second, according to new research out of Florida State University in Tallahassee.
The fixation on a beautiful face happens so quickly that it's involuntary, meaning the poor bloke who's about to get clobbered by his wife for checking out another woman, or vice versa, had no control over that initial impulse to stare at someone else.
"It's happening way too quickly for us to have any conscious control over it," said Jon Maner, associate professor of psychology at Florida State, lead author of the study, "Can't Take My Eyes off You: Attentional Adhesion to Mates and Rivals, which was published in the September issue of the Journal of Personality and Social Psychology.
But, that excuse expires very quickly, say, in about 100 milliseconds. After that, Maner said, consciousness should take over. So, this research isn't going to get anybody off the hook.
Maner and three graduate students, Matthew Gailliot, D. Aaron Rouby and Saul Miller, ran three experiments to measure something scientists call attentional adhesion. That's the length of time something holds our attention, even if we know we're supposed to be looking at something else.
More than 440 students at the university took part in the experiments, so, this is a young crowd and not necessarily representative of society as a whole, but Maner suspects it might be pretty close.
They found that participants who had been primed to think about sex and mating — by writing an essay about a time when they felt extremely sexually aroused &$151; took longer to divert their gaze from a beautiful face of the opposite sex than they did from an average face.
"We were able to show that, within a blink of an eye, people were able to physically latch on to the attractive faces," Maner said.
But when they carried the experiments a step further, they came up with a very different result.
When participants were primed to think about an event that made them sexually jealous, they fixated on an attractive face of the same sex, not the opposite.
Those two results, Maner noted, are typical of all members of the animal kingdom that reproduce by mating. Everyone wants to get the best mate possible, and everyone wants to be sure no one else horns in. Hence, it's important to keep an eye on the competition.
There's one weakness in the research, in that it's impossible to control all the variables in a crowded room. So, the researchers turned to the virtual world of computers for their experiments.
Participants were "emotionally primed" either to be sexually aroused or jealous. Then, they sat in front of a computer terminal as various images flashed on different areas of the screen.
The rules called for participants to shift their gaze immediately to anything new that appeared on the screen. For example, a picture of a beautiful face could appear in a lower corner of the screen, and less than a second later, a circle or a square could appear elsewhere on the screen, just as the face disappeared.
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.