Why are the Tau so resistant to Chaos?

The tau are not resistant to Chaos they are simply less connected to it. if you were to draw a scale from normal human to psykers to farseers of "connection to the warp" then Tau would be off the scale towards the human end meaning they have little connection to the warp and thus rarely get corrupted by chaos. This can also be seen in the fact they have no known natural psykers among their ranks.

As for why they are so unconnected to the warp the simple answer is that we don't know. it's possible that it's simply how they evolved but a lot of fan theories give one reason or another (my personal favourite being that they are related to the necrontyr.) but we won't know for sure till GW gives us a definite reason.

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