Skeptics have long argued that the human propensity to believe in gods is due to a pervasive and potent feature of human psychology–the tendency to project human form and activity. The Greek philosopher Xenophanes, maybe the first critic of religion, noted over 2500 years ago that people’s gods looked and acted like them. Ethiopians worshipped gods with snub noses and curly hair. Red haired and blue eyed people had gods with red hair and blue eyes. Warlike people have warrior gods. If oxen and horses had hands and could fashion images, said Xenophanes, their idols would be oxen and horses. The Greek gods were argumentative, boisterous, and bawdy, not unlike the Greeks themselves. Yahweh, the God of the Bible, is just as anthropomorphic as all the others: He (and he is definitely a “he”) smites, rewards, issues orders, changes his mind, suffers from bouts of jealousy and temper, booms in a loud voice, impregnates a human female, and walks in the garden in the cool of the day. So, the gods, including God, seem originally to have been merely a projection of the human tendency to anthropomorphize, to see natural occurrences as effects of powerful humanlike agents.
Theists counter that such an argument, if taken as supporting atheism, commits the “genetic fallacy.” You commit the genetic fallacy when you conflate two questions that should be distinguished: (a) What causal processes account for the psychological origins of a belief? (b) What rational grounds are there for thinking the belief true? Just because you can explain why somebody holds a certain belief (he learned it from his mother, say) doesn’t mean that the belief has no objective truth or validity. I might be “hardwired” to think that God exists, but, nevertheless, he might really exist, as arguments and evidence might show. As the saying goes, just because you are paranoid does not mean the people are not out to get you; likewise, just because you are wired to believe in God does not mean that God does not exist (Maybe, in fact, it was God who wired you to believe in him!).
However, the charge that atheists commit the genetic fallacy is both wrongheaded and disingenuous. Sometimes, indeed, the causal history of a belief has no bearing on its credibility: I may have originally accepted the Pythagorean Theorem because my high school geometry teacher pounded it into my reluctant head, but if I can now prove it, the history of how I acquired my beliefs about the Pythagorean Theorem is irrelevant to my current judgment about its soundness. On the other hand, there are times when the causal history of a belief is highly relevant to its epistemic merits. A belief acquired by the ordinary functioning of human sense organs in the appropriate circumstances (e.g., believing that someone, Bill Clinton, say, is present because he is seen from nearby in full daylight and with nothing in the way) is clearly more credible than one acquired by hallucination. If a friend, known to be trustworthy, told us that he just saw Bill Clinton walking down the street, and we believed his cognitive and sensory functions were normal, we would probably accept that Bill Clinton was in the area. But if we knew that our friend suffered a peculiar psychological condition that made him prone to Bill Clinton-hallucinations, we would strongly discount the claim that Bill Clinton was in the vicinity. Likewise, if we identified in the human psyche a powerful mechanism that inclines people to believe in gods—whether or not gods actually exist—we should, absent strong reasons to the contrary, discount belief in gods.
The theistic accusation is also disingenuous. Everyone disregards all sorts of ideas for no other reason than that we know how those ideas came about. Suppose that there are some fanatical J.R.R. Tolkien fans out there who think that Hobbits really exist and are even combatively aggressive in asserting such. Do we have a responsibility to take the Hobbit-believers’ claim seriously? Can you disprove the existence of Hobbits? I don’t think so. The reason why nobody, or hardly anybody, takes the actual existence of Hobbits seriously is that we all know where the idea of Hobbits came from. Tolkien just made them up. If Hobbit-believers accused us of committing the genetic fallacy, conflating the question of where the idea of Hobbits came from with the question of their actual existence, we would just laugh at them. Likewise for any latter-day Zeus or Odin worshippers; we know that Zeus and Odin are products of folklore and mythology, i.e., that they are just made up, and we are under no burden to separately consider the question of their actual existence. Theists find the comparison odious, but really there is no obvious reason why we shouldn’t regard Yahweh as we do Zeus and Odin.