The Institute for American Values gives us the news that -- all right, they don't say it in so many words, but two and two are easy to add, and there seems no way not to understand that David Letterman didn't inadvertently become a cad and a bounder and Roman Polanski a rapist on the run.

Our culture's grasp of the right relationship between men and women --founded on a proper understanding of marriage -- is in sorry shape.

New York City-based IAV, which studies and comments on the state of the culture, reported the other day on the problems faced by our basic civilizing institution, the family. The report packs no surprises, at least for Americans old enough to have glumly watched the steady erosion of valuable inhibitions on wrong behavior over some years.

Maybe "glumly" isn't the right word. I'll wager Letterman wasn't glum as moral barriers to the exercise of raw appetite started crumbling around the time he was in high school -- the mid-'60s -- greatly broadening the market for cads and other non-respecters of women. What people used to expect in the way of male behavior toward women, women quit expecting and males quit delivering, as nobody expected it anymore.

Our current problem: a lack of norms anyone feels obliged to recognize in this present culture of liberation; hence weaker and weaker family structures, just as the IAV report shows. The report, to be found at www.americanvalues.org, has a quasi-sociological name, the Index of Leading Marriage Indicators (which is a play on the Index of Leading Economic Indicators). The authors measure the state of marriage. They dislike the findings.

For instance: Whereas 78.6 percent of Americans, ages 20 to 54, were married in 1970, only 57.2 percent are married today.

The drop-off, during the same period, in number of intact first marriages is steeper still -- from 77.5 percent to 61.2 percent. Do the math: that's three in five. Unsurprisingly, births to married parents, which were nine in every 10 back in 1970, are now just three in five. There's more, but you get the drift: What I referred to earlier as our basic civilizing institution is weakening. That means increases in non-civilized behavior.

Granted, the un-noblest Roman of them all didn't invent teenage rape, nor was Letterman the first lecher to carry on fancy-free office affairs. The withdrawal of cultural stigmas, unfortunately, makes those behaviors more common, the victims more numerous.

Back to the family. Where do we learn, if we're going to learn at all, about such things as cultural stigmas, or, to put it another way, about rights and responsibilities, duties and obligations, do's and don'ts? We learn in the context of family life -- from Mom, from Dad, from grandparents and siblings and cousins. The family is a school that never lets out. It can be a bad or defective school. It can be a place of love and warmth. Either way it teaches. All the more usefully, it teaches if parents believe themselves to be living out a worthy idea for the shaping of human destinies, an idea grounded most of the time in mutual promises and commitments. Better, perhaps, to see commitments breached -- because they can always be repaired -- than never to see them made at all because they seemed outdated or restrictive.

The IAV report observes that when marriage works, "it creates a context in which children can flourish." You see the truth here not just in terms of sexual behavior but at all levels of life. Why isn't the education system any better than it is? Isn't it because relatively fewer parents now than half a century ago understand their family responsibilities to include prodding the young 'uns toward achievement? IAV (which collaborated on the study with the National Center on African American Marriages and Parenting) insists that "the status of our marriages influences our well-being at least as much as the state of our finances." It's Reason No. 1 on a Letterman-style list for rebuilding what we've so long allowed to decay.