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While rats are among the few people who take ideas seriously enough that just making a reasoned argument to do something has a chance to actually make them do it, you might have trouble taking that route since a lot of rats have thought about it and have their own ideas about whether the rationalist birthrate should rise or not.
Probably more plausible is to promote rationalism in areas with higher birthrates. Not likely to recruit any subsaharan africans to a subculure with an average IQ in the 140s, but there's probably groups you could target that breed faster than the existing demography of WEIRDos, atheist Jews, and autogynephilic transwomen.