Rob Arnott, founder of Research Affiliates LLC, published a paper earlier this month in which he questions the use of some “smart beta” investment strategies he pioneered, according to a recent article in The Wall Street Journal.

Arnott, known by many as the “godfather of smart beta,” says the popularity of some passive-investing strategies are making them expensive, and has “built tools that he says will help investors determine which strategies are overpriced.”

In the paper, titled “How Can ‘Smart Bet’ Go Horribly Wrong?” Arnott singles out low-volatility funds, saying that inflows have driven up valuations which, in turn, are harming future return prospects. The motivation for his analysis was the recent underperformance of some of his strategies and his observation that investors were pivoting to low-vol funds, even though they those funds had become relatively expensive.

Cliff Asness, co-founder of AQR Capital Management LLC and also an early designer of smart-beta strategies, challenges Arnott’s stance, the article says, arguing that valuations are not the only factor at play.