Matt got in touch with me and pointed me to his post where he sees the same momentum peak around the '70s as I did when I looked.

He also analyses a bunch of international equity markets and has an interesting hypothesis -

Technical Analysis pretty rapidly killed off trends in the US during the 70s.

Reminds me of theories that aborigines hunted and killed most of Australia's indigenous wildlife when they landed.

On the bright side, there's always momentum's less well known cousin to hunt out - mean reversion.