WASHINGTON - The institution of marriage in the United States has steadily declined in strength over the last four decades, according to a report released last month by a panel of scholars and advocates.

The U.S. Marriage Index, the brainchild of David Blankenhorn, president of the Institute for American Values, seeks to quantify the health of marriage in the United States in the same way economists use leading indicators to parse the state of the country's economy.

"We're just proposing a way of numerically capturing these trends so that people can see them," Blankenhorn says.

The index combined five statistics - the percentage of adults between ages 20 and 54 who are married, the percentage of adults who reported being "very happy" with their marriages, the percentage of first marriages intact, the percentage of births to married parents, and the percentage of children living with their own married parents - to reach a composite score illustrating the state of U.S. nuptial unions. In 1970, that score was 76.2; by 2008, it had dropped to 60.3.

Almost 90 percent of children were born to married parents in 1970; last year, the figure was 60 percent. Of adults between ages 20 and 54, 78.6 percent were married in 1970, compared with 57.2 percent in 2008. The portion of first marriages that remained intact dropped from 77.4 percent in 1970 to 61.2 percent last year.

Blankenhorn says the index was born partly out of his frustration with the difficulty of talking publicly about the subject of marriage.

"There's a lot of genuine opinion out there that really marriage is something that we ought to leave to people's private decision-making and it's not society's business to get into," he concedes. "You're going into their bedroom. You're going into their private lifestyle choices. You're going into situations you can't possibly understand."

Blankenhorn takes issue with that viewpoint largely because marriage has such a significant impact on children. He points to statistics showing that children who grow up in homes where their parents are married to each other are, on average, less likely to live in poverty, to have emotional or behavioral problems, to engage in premature sexual activity, to use drugs, or to commit suicide.

"Every single pathology or problem or difficulty a child can experience - every single one - growing up outside of a married-couple home elevates the risk," he says.

Blankenhorn's hope is that the index, a collaborative effort by 15 academics, researchers, and policy experts intended for release every other year, will become a bellwether signaling the direction that marriage is headed in the United States - and that it will galvanize concern and support for the institution.

"It's impossible, really, to make progress unless you have some shared understanding," he says. "There's no disagreement among us about high rates of unemployment - nobody runs around saying it's fine to have 20 percent of us unemployed. But we really are not at that level of agreement about marriage."

Blankenhorn says increases in divorce and in out-of-wedlock childbirth are the two factors that contributed most to the decline in the health of marriage in the last half century. The index also includes 101 suggestions to strengthen marriage in America, written by Blankenhorn and collaborator Linda Malone-Colon of Hampton University in Virginia. Among them: creating community-based marriage-mentoring programs and encouraging government funding of marriage education.

"All we're saying here is, can we just think about that for a minute? Can we do better?" he says. "And would we do better as a country if we did better on this?"