Why can you “double for less” but not “split for less” at blackjack?

Question: In blackjack, you're allowed to "double for less" than your initial bet but not to "split for less." Why is this?

Answer: Doubling down involves making a second bet after seeing your initial hand and the dealer's up-card, then drawing exactly once. Made properly, the extra wager boosts your expected profit. But, odds of winning the hand drop in cases when you would draw again if you were able to do so. Examples would be doubling on nine vs four-up and drawing a deuce, or on 11 vs 10-up and pulling an ace. Doubling for less helps the house by keeping the single-card constraint without granting full benefit of the auxiliary bet.

Splitting involves dividing a pair into two hands and playing each side separately – with its own bet. Some splits offer players an absolute advantage. For instance, breaking a pair of eights against a seven-up. A casino might allow splitting for less in such cases because doing so would raise its edge.

But consider a hand like nine-nine against nine-up. Your 18 is weak. Standing is expected to cost you 18 cents per dollar bet. Splitting, you're still the underdog, but the penalty on each side is only 5 cents per dollar. So, say you started with a $10 bet. Standing, your expected loss is $1.80. Splitting normally, your expected loss drops to $0.50 on each side or $1.00 total. What if you split for less – $10 on the first eight and $1 on the second? Your negative expectation would be $0.50 on the $10 and $0.05 on the $1, or $0.55 in all. A casino that offered splitting for less on all pairs would lose money on the game.