How many American marriages end in divorce? One in two, if you believe the statistic endlessly repeated in news media reports, academic papers and campaign speeches.

The figure is based on a simple -- and flawed -- calculation: the annual marriage rate per 1,000 people compared with the annual divorce rate. In 2003, for example, the most recent year for which data is available, there were 7.5 marriages per 1,000 people and 3.8 divorces, according to the National Center for Health Statistics.

But researchers say that this is misleading because the people who are divorcing in any given year are not the same as those who are marrying, and that the statistic is virtually useless in understanding divorce rates. In fact, they say, studies find that the divorce rate in the United States has never reached one in every two marriages, and new research suggests that, with rates now declining, it probably never will.

The method preferred by social scientists in determining the divorce rate is to calculate how many people who have ever married subsequently divorced. Counted that way, the rate has never exceeded about 41 percent, researchers say. Although sharply rising rates in the 1970's led some to project that the number would keep increasing, the rate has instead begun to inch downward.