Before we get into the specifics on why lines move, let’s clarify the definition of line movement. When people use the terms lines in relation to sports betting, they are generally referring to odds, or specific lines, such as the spread or over/under for a game. For example, if the Green Bay Packers are -3.5 (-110) for a game, then the spread is -3.5 and odds are -110.
As you probably have seen though, these lines are fluid and can change throughout the course of the week. The lines that are first released days before the game will likely change multiple times before the game begins. This is due to a variety of reasons.
In general, but not always, sportsbooks like to have equal money on each side of a wager. This guarantees them a profit. If the Packers are -3.5, and the sportsbook has $300,000 wagered on each side of this line, then the sportsbook is guaranteed a profit equal to its vig. Assuming a standard vig of 10%, in this example, the sportsbook would earn a profit of $30,000 from this game. If you completed our sports betting course, you also know that there are several situations where a sportsbook may not necessarily want equal money on each side of the wager.
So why do lines move? The first reason lines move is to attempt to even out the action on both sides of a wager. If a line of Browns +4 is released, and 90% of the money is being wagered on the opposite side, then the sportsbooks may change that line to +4.5 or +5 in order to get more action on the Browns. That will reduce the sportsbooks liability in case the Browns don’t cover. Line movement can also occur due to external factors such as a player’s injury, or if it is announced that a specific player is suspended or resting that game.
Lines can also move when certain sharps take one side of a wager. A lot of sportsbooks and casinos know who the sharps are, and when one or several of them place large wagers on the same side of a bet, the sportsbooks may believe that their lines are off, and adjust them accordingly. This can also result in reverse line movement, which is when the line moves in contradiction to the public betting percentage.
For outdoor sports such as baseball or football, line movements can also come as a result of the weather. If the forecast call for a significant amount of wind or precipitation during the game, that can affect the over/under. So as you can see, line movement can happen for a variety of reasons, and successful bettors can use this to their advantage if they can anticipate these movements, or find value before or after a line moves. We do offer a podcast as well that discusses strategies on how to use line movement to guide your betting decisions.