Marriage as an institution is not what it used to be. Since the 1950s, the number of couple exchanging “I dos” has dropped steadily. And while most Americans do marry at some point in their lives, many are choosing to do so later. A new study examines Americans’ changing sensibilities about marriage, using economics as a measuring tool. Researchers contend that families with high incomes and high levels of education have the greatest incentives to maintain long-term relationships.