Terrific article highlighting the similarities between belief and disbelief. Disbelief is not something you actively choose. I love how the author compares it to homosexuality. Just as once you are old enough to have those types of feelings, you don't choose to be gay or not gay, you just are one or the other. For religion, although it's not quite apples to apples, Once you've been indoctrinated one way or have gained enough scientific knowledge the other way, you cannot choose whether you believe or not believe in a personal god. You either do or you don't, you can't fake it (to yourself).
If Richard Dawkins, perhaps the world's best-known atheist, had been born in the thirteenth century, chances are he would have been theistic, believing in one kind of god or another. But, having been born in the twentieth century, having experienced his life as he has, can it really be said that Dawkins chooses to be an atheist? His status as a nonbeliever is a result of his biological composition (particularly his brain function) combined with the knowledge he has gained through his life experiences. It really is not a choice at all.
If more individuals today are religious skeptics than in centuries past, that is mainly because accumulated knowledge has inclined more people toward such doubt. As Dawkins himself has said, it would have been harder to be an atheist hundreds of years ago, when so many mysteries about the universe had not been answered. Though skepticism has always existed (the history of religious skepticism is covered wonderfully in Jennifer Michael Hecht's Doubt: A History), the scientific discoveries of the last few hundred years have filled in so many gaps that the idea of a Grand Designer with some kind of special affection for humans seems more implausible than ever to many.
But this does not mean that today's religious skeptics choose not to believe. Instead, we can see that personal secularity is primarily the result of brain function combined with access to knowledge, information, and a social setting allowing disbelief. Given the right conditions, the result will be an individual who does not accept supernaturalexplanations.
Interestingly, we can see that in many ways believers don't really choose either, but when we consider theistic beliefs we see different causal environmental factors at work. Early childhood indoctrination by family, for example, is a key environmental factor that promotes such beliefs in many, as is the pro-religion conditioning that one receives from the community and broader society. Even if the overt promotion of religiosity by society is mild (which usually isn't the case in much of America), prevailing social views that disapprove of open disbelief will often discourage serious exploration of secularity.
Thus, although causation is always complex and the specifics are going to vary from one individual to the next, in general we find two interesting patterns with regard to the formation of religious belief and disbelief. That is, the major environmental factor that promotes disbelief (and discourages belief) tends to be accumulated knowledge, whereas the most significant environmental factor in promoting belief (and discouraging disbelief) tends to be family and social indoctrination.