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> After he died, Secretariat’s heart was reportedly measured at almost 10 kilograms – twice as much as normal. This appears to have been a physiological fluke, however, as it isn’t a trait reliably passed down to offspring.

In humans, enlargement of internal organs (including heart) is know to be a side effect of using growth hormone as a PED. Wiki says [0] that "Extracted growth hormone was used since the late 1950s", so in 1973 it was already a well known technology.

[0] https://en.wikipedia.org/wiki/Growth_hormone_therapy#Extract...

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> Occam’s razor requires us to consider the possibility they just had a really good, well timed dosage of performance enhancing drugs for Secretariat back in the day.

No, Occam's Razor suggests the simplest possible explanation. To suggest that they figured out how to dope horses in 1973 and then promptly forgot how for the next fifty years is a complicated explanation. The simplest explanation, the Occam's Razor explanation, would just be that it was a fluke of nature.

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> and then promptly forgot

Maybe anti-doping rules enforcement became stricter?

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