system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout] What determines whether a person becomes a ghoul versus just being irradiated all to hell?
gpt
It does seem to mostly come down to radiation, plus an unidentified random genetic factor. While the Forced Evolutionary Virus may also be a factor, the existence of ghouls in circumstances where they could not have been exposed (like the Chinese Sub ghouls and the pre-war criminal who deliberately ghoul'd himself to survive the apocalypse in Fallout 4) suggests that ghoulification is a natural consequence of radiation in the Fallout verse. Additionally, there seems to be no distinction on race or nation, as we see Americans, Mexicans, and Chinese folk who have all been turned (with the majority being Americans purely by dint of being set in America).

There also seems to be flexibility in the timing. The ghouls of Bakersfield were exposed to lingering fallout because (by design) the door of their Valut failed to maintain a proper gas seal, while Moira of Megaton and the NCR troopers of Camp Searchlight were turned by being in proximity to a detonating nuke and an improvised radiological attack (respectively). It's possible for a ghoul to be formed by a relatively slow process (Raul effectively died slow of what looked like conventional radiation poisoning before getting back up), or to go from human to ghoul in a matter of hours (Moira can be found almost immediately after the Megaton detonation already notably transformed).