Given with Kelly resize betting: Betting over double Kelly bankroll shrinks. Betting double Kelly bankroll fluctuates with no long term growth Betting 1.5 Kelly vs .5 Kelly both have the same growth rate, with 1.5 Kelly having much more variance. So betting under Kelly is always preferred to betting over Kelly. So one is facing a game/investment where they are unsure of the advantage or variance. Perhaps even a new player to a known game like blackjack with card counting, but are unsure of their skill or the validity of card counting. What should one do? Perhaps bet from 1/4 to 1/8 Kelly resizing* to protect the bank from errors in estimating advantage. As samples from playing come in if one is winning it is a weak confirmation that one does have an advantage and can bet more. If one is losing they bet less until wins proves a winning strategy. In brief: Underbetting; even by a large margin, is preferred to overbetting. However, its your money to bet as you want. *For the aspiring or even established pro 1/4 Kelly resizing is quite conservative for blackjack with 1/6 to 1/8 Kelly being used for the unsure game.