This has never been shown in a controlled experiment, all we have are anecdotes.
There wasn't a real experiment, but the number of cases trends to be enormous, and their ressemblence can't be a coincidence.
I've never been a remotely 'spiritual' person. I think it's mostly due to my love of science as a kid (Carl Sagan was my hero), but I wonder if there's some natural variation our brains that predisposes a significant portion of the population to spiritual/religious beliefs regardless of upbringing. I know it's been suggested that religiosity is tied to OCD and the schizophrenia range of mental disorders.
Predisposition for spirituality or religion isn't genetic. We saw families of the same religion because they live in the same context.
And I claim one thing : "do not confound religion and spirituality", it is like confounding a fact and its interpretation.