which is more common

[mechanical]

This is something I've been wondering about and I can't think of a better place to ask. Which do you think is more common, a mother having sex with her son or a father having sex with his daughter?
It seems to me that it would be more common for a mother son relationship since a father has to be so careful about trying to start a relationship with his daughter, but since mothers as a rule always seem to be more understanding and forgiving of their sons and teen boys always being enamored...

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