But in all this enthusiasm, one group has been largely ignored. "People who truly have no religion are not very well understood," says David Yamane of Wake Forest University, editor of the journal Sociology of Religion.
The few studies that did treat nonbelief seriously offered tantalizing hints that to look only at religiosity was to miss an important part of the spectrum of human belief. One study conducted in 1985 by German psychologist Franz Buggle and his colleagues suggests that neither religion nor irreligion has a monopoly on improving people's mental health. Among 174 people surveyed, it found that two groups enjoyed the lowest scores on a scale of depression: the most pious Christians and the convinced atheists. Those in the middle, the lukewarm believers, were most likely to be depressed. In 2005, a team at Newcastle University in Britain reported a similar result.
More recently, Karen Hwang, a professor at the University of Medicine and Dentistry of New Jersey, decided to examine atheists at risk for depression more closely. Hwang's interviews with atheists suffering from spinal cord injuries revealed how becoming debilitated strengthened their convictions, and their convictions strengthened them. "It doesn't matter so much what a person believes in," she says, "but how consistent and cohesive their worldview is."
Wulff has been developing survey tools that will help psychologists look beyond binary oppositions like religiosity and secularity, or belief and unbelief. Phil Zuckerman's study in Scandinavia, in fact, suggests that these distinctions aren't as clear as one might expect. His interviews show the extent to which, even in the absence of traditional supernatural beliefs, the subjects' religious heritage provides them with moral guideposts and cultural habits. Not believing in God doesn't stop most Danes and Swedes from considering themselves Christians.
Religions, we are beginning to learn, can be better understood by paying attention to what irreligion looks like. Probe irreligion, and you encounter not only new insights about how it works in people's lives, but also echoes of the very religions it defines itself against.