There's no such thing as an atheist baby

[Act_of_Reparation]

33. It isn't weak at all.

The comparison is functionally appropriate. The reason no one self-identifies as a not-stamp collector is because society hasn't imposed distinctions between those who collect stamps and those who do not. People do not assume not-stamp collectors are inherently immoral and untrustworthy, that not-stamp collectors are un-American, or that not-stamp collectors are "just as bad" as stamp collectors. Not-stamp collectors, generally speaking, aren't prone to feeling isolated or ostracized; they don't feel the need to commune with other not-stamp collectors. None of this has any bearing on the comparison, however, which is all about demonstrating that not believing in gods can be as passive a position as not partaking in a niche hobby, sport, or interest.

So, I would argue that in some cases atheism is really an active position.

Whether or not one self-identifies as an atheist has nothing whatsoever to do with whether or not the belief is "active" or "passive". As others have already said, the distinction comes down to whether one is a gnostic or agnostic atheist.

If we treat the question of the existence of gods like a scientific hypothesis, it would look like this:

Hypothesis (+): There is evidence god or gods exist.
Hypothesis (-): There is evidence god or gods do not exist.
Hypothesis (Null): There is no evidence for the existence or non existence of god or gods.

Gnostic atheists disbelieve because they think they have sufficient evidence to disprove the existence of god or gods. This is a constructed position assembled from empiricism, logic, personal experience, what have you. One must actively assume this position.

An agnostic atheist, on the other hand, disbelieves because there isn't sufficient evidence to believe. It is the default position in an existential argument. It is a passive belief, regardless of how one self-identifies. It is no more reasonable to think people actively disbelieve God than it is reasonable to think Christians actively disbelieve Xipe Totec, or Ahura Mazda, or any of the hundreds of thousands of deities they've never heard of.

Also it was mentioned above that twin studies have shown this to be the case, though I have not seen these studies.

Twins share the same genetic information. If one maintains the family religion while the other does not, then it would seem religious preference is a product of socialization, not genetics.