Why are female serial killers so few in number?

Is it that they are less likely to be caught due to being seen as more 'innocent' throughout much of history, is it instead that they are simply less likely to commit serial murder due to different brain chemistry, or is it that their preferred methods are much easier to disguise compared to male killers?

For example, many of the documented female killers or nurses or noblewomen. Nurses can just claim natural causes if they commit the act say via suffocation or overdose - especially back in the day when things were harder to prove - and noblewomen would have the position of power to silence anyone under them. Despite that, if they go far enough the word will eventually get out, as is the case with those such as Elizabeth Bathory, Delphine LaLaurie or La Quintrala.

So, what do you think? Are women simply less prone to the psychological makeup that makes some a killer, or do differences in temperament make it easier for them to get away with it?
Why are female serial killers so few in number?
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