In many cases, Scottish and Japanese researchers say, the key to attraction is in the time of a woman's cycle.
According to the study, when a woman is ovulating, or ready to conceive, she is likely to prefer men with more masculine features. When she is menstruating, or least likely to get pregnant, she is apt to prefer softer, more feminine looks.
The researchers believe this is not a 20th century standard of beauty, but something that is instilled by evolution for sound biological reasons: In the animal kingdom, masculine looks denote virility, and thus the ability to produce healthy offspring.
"Literature from the animal kingdom suggests exaggerated male features such as peacock feathers and deer antler and other masculine features display a good immune system, and that's what should be atttractive for females," David Perrett of the University of St. Andrews in Scotland, one of the study's authors, told the Associated Press. "And to some extent we are finding that's what's true for humans."
"They are interesting observations and certainly go along with what we already know," says Dr. Lauren Streicher of Northwestern Memorial Hospital, who was not part of the research team.
What doctors know is that when a woman ovulates, nature's goal is to produce a baby. Researchers think women prefer a more masculine type of face because he presumably has more testosterone and therefore the best chance of conceiving a baby.
The findings build on work by the researchers last year that suggests women overall prefer male faces that are slightly feminized. The scientists speculate that subconsciously, women see them as more like women, more nurturing, more responsible -- a better child-rearing partner for the long haul.
Researchers note, of course, that biology is not the only thing that governs our choice of a mate. Social and cultural factors do, too: earning power, common interests and similar backgrounds, for example.
In the study, women in Japan and Scotland were shown computer-generated male faces that had been altered in their level of masculinity -- including the size of the jaw, eyebrow ridge, and overall size.
All of these masculine features are caused by testosterone, the male hormone. Similarly, in women, estrogen causes breasts, fuller hips, a smaller chin and full lips -- all features that denote fertility and are generally considered most appealing to men.
The 39 Japanese women were questioned about their menstrual cycles and later were shown five white and five Japanese faces on two occasions, when they were most likely and least likely to become pregnant.
For Japanese and white faces, the women preferred faces that were on average about 20 percent and 15 percent feminized, respectvely, when they were least likely to become pregnant.
When they were most likely to become pregnant, the women preferred faces that were only about 8 percent feminized for both groups of images.
In the second experiment, 65 British women were asked over a three- to four-week period to choose the most attractive face for long-term and short-term sexual relationships. The women used a computer that allowed them to make a composite face more or less feminine.
For short-term relationships, the preferred faces were about 15 percent feminized during low-conception risk periods, vs. about 8 percent during high-conception risk periods. For long-term relationships, the preferred faces were about 10 percent feminized throughout the month.
Women on the pill, which controls the monthly hormonal cycle, were excluded from the Japanese study. But in the British experiment, these women picked a certain kind of face without varying their preferences.
The findings suggest at least one piece of practical dating advice: A man who gets rejected by a woman might have more success if he asks her out again in a week or two.
"They should at least try for four weeks on the run," Perrett said. "It does imply some persistence is a good strategy for males."