In 1972, Dean Kelley wrote a book called "Why Conservative Churches Are Growing." Using a combination of theory and data, Kelley argued that conservative Protestant groups, such as Baptists, the Assemblies of God and the Nazarenes, were attracting members while liberal Protestant bodies, such as the Episcopalians, Presbyterians,and the United Church of Christ, were declining in membership.

In more recent years, a number of other researchers have advanced the same argument. Like Kelley, they contend that strictness fosters church growth. Demanding, counter-cultural groups are more likely to attract and retain members than lenient, culture-affirming groups.

But, why? How does strictness foster commitment and growth? According to economist Laurence Iannaccone, strictness strengthens a church in three ways: it "raise[s] overall levels of commitment," it "increase[s] average rates of participation," and it "enhance[s] the net benefits of membership."

This thesis has gained considerable acceptance among Protestant and Catholic church leaders. Embracing the strictness thesis, these leaders argue that adopting a conservative theology that requires members to set themselves apart from prevailing cultural norms and values increases a group's chances of increasing its membership and fiscal solvency. Conversely, they argue, a liberal theology that reaffirms the prevailing culture attracts fewer people and leads to financial difficulties.

There is no question that, at least in the last 50 years or so, there has been a correlation between strictness and church growth. It is easy to prove that conservative groups such as the Assemblies of God and Jehovah's Witnesses are growing, while liberal groups such as Episcopalians and Presbyterians are losing members.

But, one should not mistake correlation with causation. To show a correlation between strictness and church growth does not prove that strictness explains the growth. It might, but the correlation could be spurious, and other factors that happen to correlate with strictness might be the real reasons for the growth.

Indeed, a number of studies have shown that, when strictness is considered along with a number of other possible influences, the other factors explain more of the variation in church growth. It is not that conservative or demanding theology is irrelevant; it seems to have at least some impact. However, other factors seem to have even more effect. Let me mention two of them.

First, contextual factors related to population growth and decline in the larger community or in surrounding neighborhoods help to explain why some churches grow and others decline. Even lenient, liberal churches in rapidly growing communities and neighborhoods are likely to grow, while conservative churches in declining neighborhoods are likely to see a decline in membership. For example, the influx of new immigrants helps to explain the growing size of Catholic parishes in many communities.

Second, regardless of theology, the more churches meet prospective members' social and spiritual needs, the more likely they are to grow. It so happens that many of the churches that are especially good at discerning, and responding to, the needs of people in surrounding neighborhoods and communities have conservative theologies. But, liberal churches that address people's need for companionship, assistance in raising their children, and worship services that intensify their relationship with God also grow. Whether they have liberal or conservative theologies, churches that are unable to meet people's needs tend to fail.

In my estimation, strictness has been given far too much credit for being the factor that accounts for church growth. Contextual influences and churches' adroitness in meeting people's social and spiritual needs are even more important. Even liberal churches can grow when they are in growing communities and neighborhoods and when they address the needs of the people in their area.

James D. Davidson is professor of sociology at Purdue University in West Lafayette, Indiana. His most recent book is "Catholicism in Motion: The Church in American Society" (Liguori/Triumph, 2005).