In blackjack, why can you only hit aces once after a split?

Question: After splitting a pair in blackjack, you can normally play each side like a regular hand. The exception occurs when you split aces. Then you can only hit once on each side. Why is this?

Answer: The casino has an edge in blackjack because players must complete their hands before the dealer acts. Bettors who go over 21 therefore lose, even in rounds when dealers subsequently bust.

If winning blackjacks only paid 1-to-1, players following the same rules as the dealer – hit all hands under 17, stand on 17 and above, no splits or doubles – would have about 7.5 percent disadvantage based on the probability of simultaneous busts. This edge is reduced by about 2 percent by the 3-to-2 payoff on uncontested blackjacks. It's lowered by an additional 3 percent when players follow basic strategy, standing on appropriate totals under 17 for dealer upcards from two through six. Proper splits and doubles, together, cut about 2 percent more from the edge. Combined, this leads to players only having to fight half a percent edge – which is one reason the game is so popular.

To get to your question, a starting ace is extremely powerful. Chances of a 21 exceed 30 percent when only a single card can be drawn. If additional hits were allowed, there'd be opportunities to double down when the dealer was vulnerable, and the chances of busting following basic strategy against dealer upcards of seven or more would be under 12 percent. Letting you treat aces like other cards after splitting would therefore give players rather than the house the advantage in the game, such that casinos couldn't afford to offer it.