Rats & Batters

May 9, 2026

What do the Great Hanoi Rat Massacre of 1902 and MLB’s new ABS challenge system have in common? Both show how incentives can lead to unintended consequences.

In 1902, French colonial authorities in Hanoi offered a bounty of one cent for every tail from a dead rat to control the city’s growing rat population. The program appeared to be successful with daily tails peaking at 20,000. Eventually, officials noticed many tail-less rats throughout the city. It turned out that enterprising people were breeding rats, cutting off tails, and releasing the maimed rodents. The program was abandoned, but it was too late. In 1903, the bubonic plague arrived in Hanoi.

Baseball may be discovering a modern version of the same law of unintended consequences.

MLB has introduced a new Automated Ball-Strike (ABS) challenge system. The pitcher, catcher, or batter can challenge a call with a limit of two unsuccessful challenges per game. Season to date, 53% of challenged calls have been overturned. Interestingly, umpire accuracy, which has been measured for over two decades, improved meaningfully in 2025 on the eve of ABS adoption. Public accountability in 2026 has led to further improvement.

Not all the incentives stemming from ABS are as straightforward as umpire accuracy. Many professional athletes across sports inflate their reported heights. Since the strike zone is defined as 27% of a batter’s height at the bottom and 53.5% at the top, players suddenly have an incentive to be shorter. MLB took official measurements of every player before the season. Heights of 171 players “shrank” by an inch, 48 by two inches and 6 players by three inches!

Also, catchers who once tried to frame borderline balls to look like strikes now have the opposite incentive: make borderline strikes look like balls to induce failed challenges.

In asset management, incentive systems often reward gathering assets rather than generating returns. As in baseball and Hanoi, people respond rationally to the system they are given.

Measure what you want managed.

Incentives and Framing are two chapters in my forthcoming book, Running Against the Herd: Battling Biases to Make Better Investment Decisions, now available for pre-order.