Why Prediction Markets Have the Under Your Sportsbook Doesn't

You’ve probably run into this. You pull up a home run prop, you want the side that says this hitter doesn’t go deep tonight, and the menu only offers you the Over. There’s no Under 0.5 Home Runs to click. The market looks one-sided because it is one-sided.

Move that same question to a prediction market like Kalshi or Polymarket and the Under is always sitting right there. That isn’t a product decision either exchange made to be generous. It falls out of how the two kinds of venue are built, and it changes what the bet costs you.

A sportsbook picks its sides. An exchange can’t.

A sportsbook writes the menu. It decides which markets to open, which lines to hang, and — the part most people never think about — which sides of those markets to actually post. Nothing forces a book to offer both. If a market draws lopsided action, or the other side is a heavy favorite that pays out pennies, a book can simply not list it.

Home run props are the clearest case. Here’s what the sportsbooks we track were doing with MLB batter props on August 18, 2026:

• FanDuel listed no Under on any batter prop at all — home runs, hits, total bases, RBIs, runs, doubles, singles, triples, stolen bases, every one of them Over-only.
• DraftKings did post Unders on hits, total bases and RBIs — but not on home runs, which it listed as a ladder of Overs (1+, 2+, 3+) with no Under at any price.
• Hard Rock and theScore posted both sides.

Every other sportsbook we track — DraftKings, FanDuel, BetMGM, Bally — listed the Over on a 0.5 home run line with no Under beside it.

That spread of behavior is the point. These books see the same games and the same players, and they still make different calls about which halves of a market are worth listing. Note that FanDuel does quote both sides of MLB game totals and team home run totals — so this isn’t a book that can’t price an Under. It’s a book that chose not to post one on individual batters.

A prediction market doesn’t have that option. On an exchange you aren’t taking a price from the house — you’re buying a contract that settles at $1 if the thing happens and $0 if it doesn’t. Every contract needs someone on each end. For you to buy Yes, somebody has to be holding No. The two sides aren’t separate products the venue chooses to stock; they’re the same product viewed from opposite ends.

You can see it in the data. Across every MLB market we track on Kalshi and Polymarket, the number of Over-equivalent contracts and Under-equivalent contracts is identical — not close, identical. It can’t be otherwise.

Reading Yes/No as Over/Under

Exchanges label the sides Yes and No rather than Over and Under, which trips people up. The translation is direct: Yes on “1+ home runs” is Over 0.5. No on the same contract is Under 0.5.

Here’s a real one. On the afternoon of August 18, 2026, Polymarket US listed a contract on Ben Rice hitting one or more home runs against Baltimore that evening. The best bid to buy Yes was 20¢; the best offer was 21¢.

That single quote contains both sides. Buying Yes costs you 21¢ to win 79¢. Buying No costs 100 − 20 = 80¢ to win 20¢ — and buying No is taking the Under. Nobody had to decide to offer it. It exists because the Yes exists.

The prices also tell you what the market thinks: around 20¢ means roughly a 20% chance, which is another way of saying the Under is a heavy favorite here. That’s exactly the shape of bet sportsbooks are least eager to post.

What it actually costs

Sportsbooks charge you through vig — the margin folded into the odds, which you can see by adding both sides’ implied probabilities and watching them total more than 100%. Exchanges charge differently: a bid-ask spread (the gap between what buyers offer and sellers ask) plus an explicit trading fee.

To compare them honestly you have to put them in the same units. The fair measure is what it costs to buy both sides at once — on a book that’s the overround, on an exchange it’s the spread. Here’s what we measured on MLB player props on August 18, 2026:

Market	DraftKings	FanDuel	Hard Rock	Kalshi	Polymarket US
Home runs	Over only	Over only	5.2%	1.0%	1.0%
Hits	6.9%	Over only	7.1%	1.0%	1.0%
Total bases	6.9%	Over only	8.0%	1.0%	—
Pitcher strikeouts	6.1%	6.1%	7.5%	1.0%	1.0%

“Over only” means no Under was listed at any price, so there’s no two-sided market to measure. A dash means we saw no market at all.

One caution on that table, because it’s easy to get wrong: an exchange’s midpoint price makes it look like there’s no margin at all, since the two sides of a binary contract sum to exactly $1 by definition. That’s an artifact of using the midpoint, not evidence of free trading. The numbers above use real bid and ask quotes, which is why Kalshi and Polymarket show 1% rather than zero.

You also owe a fee on top. Both exchanges publish one, and as of August 2026 both use the same shape:

• Kalshi: 0.07 × contracts × price × (1 − price) per trade
• Polymarket US: 0.06 × contracts × price × (1 − price)

Why that fee shape favors the Under

Look closely at price × (1 − price). That term is largest at 50¢ and shrinks as the price moves toward either end. A coin-flip market is where the fee bites hardest; a lopsided one is where it nearly disappears.

Now put the two facts together. The Under on a 0.5 home run line is a heavy favorite — in our August 18 sample those Unders were priced around 80¢ and up. At 80¢ the fee works out near a penny per contract. At the 50¢ midpoint it would be closer to 1.5–1.75¢.

So the side that sportsbooks are least likely to post is the same side an exchange’s fee structure treats most cheaply. That’s the whole argument in one sentence, and it holds on both venues because both use the same curve.

Who’s actually on the other side

“Peer to peer” makes it sound like you’re matched against another fan who happened to want the opposite side. Usually you aren’t. Both exchanges run market maker programs — firms that commit to quoting both sides continuously, in exchange for reduced fees and higher position limits. Kalshi runs a central limit order book as a CFTC-regulated exchange and works with designated market makers who take on defined quoting obligations. Polymarket US pays a maker rebate to traders who post resting orders rather than take existing ones.

That’s why those Under quotes are there at all hours, and why the spreads are as tight as 1%. It’s professional liquidity, not a coincidence of matched fans. It’s worth knowing, because it also means you’re often trading against someone whose whole job is pricing these markets.

What you give up

Tighter pricing isn’t the only variable, and a few differences matter more than the cost gap:

• Depth is thinner. A 1% spread on the screen tells you the price of the next small trade, not that unlimited size is available there. Sportsbooks will typically take a larger bet at their posted number than an exchange order book will absorb without moving.
• Resolution rules are their own thing. These aren’t sportsbook grading rules. That Ben Rice contract, for example, settles to what Polymarket calls the last fair market price — not void, not push — if the player isn’t in the starting lineup and doesn’t record a plate appearance. Read the rules text before assuming a market behaves like the equivalent prop.
• Availability varies by state, and it’s moving. Access to sports event contracts differs depending on where you are and has changed repeatedly through 2026. Check the exchange directly rather than assuming.
• There are two Polymarkets. The CFTC-regulated US app and the global site are separate exchanges with separate order books and different liquidity. Prices on one tell you very little about the other.

None of this says an exchange is the right venue, or that any Under is a good bet. It says the Under exists there by construction, priced on a fee curve that happens to be gentlest exactly where the Under lives — and that’s worth understanding before you conclude your sportsbook simply forgot to list it.

There’s a second reason an Under can be missing that has nothing to do with the venue: the number you’re looking at may not be the main line at all. Alternate lines work the same way in every sport — the ladder rungs only go one way.

If you want to know how often an Under has actually landed rather than what it costs to take, that’s a different question and a better starting point — what a hit rate is covers how to read those numbers, and the MLB props pages show how frequently each market has gone Over or Under across recent games. To price out a full set of legs, the parlay analyzer runs the math leg by leg.