Only one in three of today's marriages can be expected to last, according to a new study that says divorce and separation have been under-reported.

The projection is contained in a study in the journal Demography, which says an apparent decline in divorces since 1980 is likely to have little effect on long-term prospects."This decline may indicate a return to a more stable family life; however, it would be foolish to jump to that conclusion from this brief deviation from the trend," said researchers Larry L. Bumpass and Teresa Castro Martin of the University of Wisconsin's Center for Demography.

With the marriage rate running about double that for divorces, they note that the basic data indicate "that 56 percent of recent first marriages would be likely to disrupt within 40 years of marriage."

However, studies have found a "serious under-reporting" of divorce and separation in the government's Current Population Survey, according to their study: "Recent Trends in Marital Disruption."

Taking that into consideration "we conclude that the best estimate based on these data is that about two-thirds of all first marriages are likely to disrupt."

"As high as this estimate may seem, it is based on the assumption that a plateau in the long-term trend has in fact occurred," the authors added.

Bumpass noted in a telephone interview that published divorce statistics do not include separations.

Population experts estimate that as many as 6 percent of marriages end in separation without a divorce, he said.