OVER the transom the other day came an urgent “Cougar Alert”: There is a new book out, and this one distinguishes the real cougar, a confident, strong, single woman over 40, from the comically desperate predator-seductress depicted in television shows like “Cougar Town,” one of the latest products of Hollywood’s obsession with the older woman.

There is so much cougar hype that we now have a fake cougar and a real cougar. We also have our first Miss Cougar USA, a 42-year-old crowned in August by a room full of “cubs,” men in their 20s and 30s. Cougar cruises are setting sail, cosmetic surgeons are promising to cougarize their clients and online cougar communities are cropping up.

Newsweek, taking stock of the explosion of on-screen romances between older women and younger men, declared 2009 “the year of the cougar,” but then concluded in the June article that “by this time next year, the cougar will be extinct.”

Maybe so — if you’re talking about television or the box office. But behind the unleashing of cougars in pop culture is what a growing number of sociologists say is a real demographic shift, driven by new choices that women over 40 are making as they redefine the concept of a suitable mate.

The loosening of relationship conventions, which is not limited to age but also includes race, religion and economic status, appears to be particularly evident among female baby boomers, sociologists say, who are faced with the tightest “marriage squeeze” — the smallest pool of compatible men as conventionally defined, those two to three years older, of similar background and higher levels of education and income. The reason is that as women have delayed marriage, men still have a tendency to date and marry younger women.

In the last several years, as the loaded term cougar was popularized by the media’s frenzied fascination with Demi Moore’s marriage to Ashton Kutcher, 15 years her junior, and the dalliances and liaisons of other celebrities like Madonna and Katie Couric, researchers have begun to examine the older woman-younger man relationship. It is one that has long been taboo, heavily influenced by the Freudian notion that the older women are mother substitutes or “robbing the cradle.”

“For a long time we’ve been fed this idea that women should look for a man to take care of her, a man that is more educated, has a better job and makes more money,” said Sandra L. Caron, a professor of family relations and human sexuality at the University of Maine. “That might be fine and dandy if you’re in high school and have this fairy tale Prince Charming. But when you look at adult women, most are self-sufficient and they don’t have to look for that.”

Dr. Caron is an author of a 2006 study of couples in which the wife is at least 10 years older, which found surprisingly positive attitudes among the couples, although fear of stigma and insecurity about aging for the women, in particular, were common.

The study, published in the Journal of Couple and Relationship Therapy, reported that the couples thought their age difference mattered more to the outside world than to them, and that the men were more strongly drawn to the relationships at the start because of physical attraction.

Consistent with most other research and what many relationship experts are saying about these connections, the authors found that women liked the vitality the younger man brought into their lives, and men liked the maturity and confidence in the women, although generational differences sometimes made both partners uncomfortable. Others have also cited infidelity as a stronger possibility in any relationship with a large age difference.

“Initially I thought I would find more issues,” said Nichole R. Proulx, the lead author of the study, who is a marriage and family therapist in Maine. “But it’s a relationship like any other, despite what society might say. I thought I’d find that he looks at her like his mother, more inequality, more power struggles.”

That study involved only eight couples, and the samples have generally been small in other research into a subject that has not until recently received much scholarly attention.

An analysis of census data on age difference in marriages showed that the number of marriages between women who are at least 5 or 10 years older than their spouses is still small, 5.4 percent and 1.3 percent, respectively. But both rates doubled between 1960 and 2007, according to Andrew A. Beveridge, a demographer at Queens College, who conducted the analysis.

At the same time, the data showed that the percentage of marriages of older men and younger women decreased steadily through 1980, and since then it has remained stable.

Sociologists say these figures reflect a solid change, if not a major shift in marriage patterns.