As Americans chase the modern romantic ideal of the soul mate, two classes of marriage continue to emerge: good and bad.

Since men and women first began marrying some 6,000 years ago in Mesopotamia, expectations for the institution continue to evolve with the modern Western notion of romantic love. Although some blame same-sex marriage and other social factors on rising divorce rates, new research from Northwestern University suggests a fundamental problem — Americans expect more from marriage but spend less time and energy making it work.

That conflict spells trouble for most marriages, says Eli Finkel, a psychology professor at the Weinberg College of Arts and Sciences at the university. "The issue isn't that Americans are expecting more versus less from their marriage, but rather that the nature of what they are expecting has changed," Finkel said. "They're asking less of their marriage regarding basic physiological and safety needs, but they're asking more of their marriage regarding higher psychological needs like the need for personal growth."

As Finkel explains, evolving attitudes toward marriage come with shifting social and economic conditions in the United States, as marriage becomes less about basic needs — for many — than higher psychological drives. In the early years of the republic, men and women married young for practical purposes, powering small farming villages where labor outside the home was rare and the marital unit provided food, shelter, and safety.

"In 1800, the idea of marrying for love was ludicrous," Finkel said. "That isn't to say that people didn't want love from their marriage — it just wasn't the point of marriage."

However, Americans began turning to romance as the country grew larger towns and cities and worried less about the basics of Maslow’s Hierarchy of Needs. "To be sure," Finkel observed, "marriage remained an economic institution, but the fundamental reason for getting married and for achieving happiness within the marriage increasingly revolved around love and companionship."

That ethos soon led to a third type of marriage as American culture changed rapidly later last century, one focussed not only on romance but in personal growth within the pairing. "In contemporary marriages, "Finkel said, "Americans look to their marriage to help them 'find themselves' and to pursue careers and other activities that facilitate the expression of their core self."

Sounding a bit like TV's Dr. Phil, Finkel says his own marriage continues to succeed as he and his wife devote time and energy to each other. "In general, if you want your marriage to help you achieve self-expression and personal growth, it's crucial to invest sufficient time and energy in the marriage. If you know that the time and energy aren't available, then it makes sense to adjust your expectations accordingly to minimize disappointment."

The study will appear later this year in Psychological Inquiry.