The rise of passive index investing has not materially reduced market efficiency because a resilient subset of information-producing active investors remains sufficient to coexist with the larger passive ecosystem.

"He is sceptical of claims that a decline in active management automatically reduces market efficiency. Most active managers, he says, are not uncovering deep mispricings but "are built around publicly available information and a belief in the power of mean reversion." The subset of genuinely information‑producing investors is small and resilient enough, in his view, to coexist with a larger passive ecosystem."