Most people believe that bookmakers attempt to "balance" their action, by adjusting their prices so that they get the same amount of money on both sides of a game. Theoretically, the bookmaker's only financial interest in the bets it accepts is the vigorish it takes from losing wagers, and it simply wants to ensure that the amount of wagers on each side is equal. In reality, however, bookmakers attempt to maximize their bottom line. While having an exactly equal amount of money wagered on each contestant would guarantee themselves a profit and eliminate their risk, that won't necessarily maximize their bottom line. They can make more money when they accept bets at odds which are "inflated" from those which are likely to occur. So for example, if the majority of their customers are going to bet on a team regardless of the price, they will set the price as high as possible. This is called "shading" the line. Generally, the public prefers to back the favorite, and unsophisticated bettors often show up during large events such as the Final Four and the Super Bowl. Some bookmakers actually offer different prices to different customers, using past bets as an indicator of who the customer will bet on as a way of additionally increasing their potential profit.

With a match offering a point spread, however, bookmakers must be careful of moving the line too much. Assume, for example, that a large number of Oklahoma betters caused the line to be moved from 27 points all the way to 29 points. If Oklahoma won the game by 28 points, the bookmaker would have to pay both those who wagered that Oklahoma would win by 27 and those who took Kansas on the 29 point spread. Bookmakers refer to such an event as "being middled." This famously occurred in the 1979 Super Bowl between the Pittsburgh Steelers and Dallas Cowboys, which American bookmakers still remember as Black Sunday. For that game, bookmakers opened Pittsburgh as a 3.5 point favorite, and the line closed just before kickoff at Pittsburgh -4.5. Pittsburgh won the game 35-31, enabling both those who took the Steelers -3.5 and those who wagered on the Cowboys +4.5 to collect.

Sometimes, a point spread is set at an amount that equals a common margin of victory for a particular sporting event. For instance, American football games are often decided by 3 points (the amount awarded for a field goal) or 7 points (the amount awarded for a touchdown with a successful extra-point attempt). In the case of a football game where the favorite is -7, moving the line up or down would likely result in a middle if the favorite wins by exactly 7 points. In this situation, the bookmaker may choose to adjust the vigorish in response to unbalanced action, rather than move the point spread. If the 7 point favorite is getting the most wagers, a bookmaker may change the vigorish on that team from -7 (-110) to -7 (-120), and move the underdog to +7 (+100). Once this occurs, bettors looking to wager on the favorite must risk $120 for every $100 they wish to win, while underdog players will get even money for every dollar they wager.

A bookmaker's line can be influenced by one or several large wagers made on a match. Bookmakers pay particular attention to the bets of a professional sports gambler, commonly known within the industry as a "sharp" or "wiseguy." Some bookmakers will not accept bets from bettors they believe fit in this category. Professionals evade such efforts by hiring agents known as "beards" to place bets on their behalf. Groups of professionals who work together are known as a "syndicate." These syndicates will often place large wagers with several books simultaneously, causing the prices to move quickly. Observers refer to these fast line movements as "steam."

Conversely, bettors who are primarily recreational are referred to as "squares". Online, there are certain betting shops that cater more towards sharps and those toward squares. Shops that cater towards professionals generally have higher (or no) upper betting limits and offer lower vigorish, while making some of the money back on fees for withdrawals or minimum bets. Meanwhile, "square" shops generally have lower betting limits and offer more signup bonuses. In return, they charge the standard 11-to-10 vigorish, and offer worse moneylines than the "sharp" shops. In many of the minor sports, sharps make up the majority of bettors, while for large public sporting events such as the NCAA Men's Division I Basketball Championship and the Super Bowl, recreational bettors make up almost 90% of the betting action at sportsbooks, and are the top betting events both in Nevada and online.[1]

Because of how lines move quickly during sporting events, arbitrage betting is possible. Theoretically, this will guarantee a small profit of 3-6% when a person bets on one line at one shop and on the opposite line at another shop. However, a large sum of capital is required for the amount of reward, and great care must be exercised to avoid accidentally betting on the same side at both shops.

Arbitrage situations are commonly found during halftime and intermission periods, where there is a limited amount of time for each bookmaker to determine the line and accept bets. Arbitrage betting is also possible in the weeks and days prior to sporting events as betting lines, especially for American football, are generally set early in a given time period and then adjusted in reaction to betting patterns and other relevant information as the time of the game approaches. Savvy gamblers will sometimes be able to place an early bet using the intial line and a late bet using the later line and then hope that the actual result of the contest falls somewhere between the two bets, thus ensuring either a large win or zero losses.

With the advent of online gambling came odds comparison sites that highlighted arbitrage opportunities by displaying the realtime prices of a number of bookmakers alongside each other. These sites continue to provide the most crucial information to professionals and casual gamblers alike who are looking to bet at the best odds.

In general, all forms of arbitrage betting is commonly known as "middling" or "finding the middle" of a particular contest and were fairly common in the early days of organized sports gambling. However, the relatively recent advent of both better communications between the individual sports books, the internet, and more sophisticated linesmaking techniques, has led to fewer opportunities for gamblers to make these types of bets.

The Federal Wire Act of 1961 was an attempt by the US government to prevent illegal bookmaking.

Federal Wire Act

The Interstate Wire Act of 1961, often called the Federal Wire Act, is a United States federal law prohibiting the operation of certain types of betting businesses in the United States. It begins with the text:

Whoever being engaged in the business of betting or wagering knowingly uses a wire communication facility for the transmission in interstate or foreign commerce of bets or wagers or information assisting in the placing of bets or wagers on any sporting event or contest, or for the transmission of a wire communication which entitles the recipient to receive money or credit as a result of bets or wagers, or for information assisting in the placing of bets or wagers, shall be fined under this title or imprisoned not more than two years, or both.

The law has been interpreted by some, including the Department of Justice, to mean that all online gambling is illegal. However, U.S. Courts have ruled to the contrary. Also, many believe the phrase "in the business of" means only businesses are affected. Some argue that the law only covers sports betting, and not other forms of gambling such as poker.

The U.S. Fifth Circuit Court of Appeals has ruled that the Wire Act applies only to sports betting and not other types of online gambling. The Supreme Court has not ruled on the meaning of the Federal Wire Act as it pertains to online gambling.

Betting scandals

Historically, sports betting has been associated with a number of unsavory characters, which has a lot to do with its desultory legal treatment throughout the world. Organized crime notoriously has relied upon sports betting for money laundering or funding purposes. The corruption or threat of a boxer to take a dive at the x round is a frequent theme in mafia-related movies. All of the American professional sports leagues, as well as the National College Athletic Association (NCAA), take stringent measures to disassociate themselves from sports gambling. All of these leagues, especially the NFL, make a great amount of effort to be as transparent as possible with the public, particularly with regards to injuries of players, so as to prevent any opportunity or incentive for the appearance of impropriety or collusion with the sports gambling industry.

Nevertheless, sports history is riddled with several incidents of athletes conspiring with gamblers to fix the outcomes of sporting events, or criminals acting against athletes whose on-field performance affected their wagers.

In 1919, gamblers bribed several members of the Chicago White Sox to throw the World Series. This became known as the Black Sox Scandal and was recounted in book and movie form as "Eight Men Out".
In 1978, mobsters connected with the New York Lucchese crime family, among them Henry Hill and Jimmy Burke, organized a point shaving scheme with key members of the Boston College basketball team.
On August 24, 1989, former baseball player Pete Rose voluntarily accepted a permanent ban from Major League Baseball for allegedly betting on Reds games while managing the team. However, the bets he made were only for his team to win.

Andrés Escobar, a Colombian defender, was murdered shortly after his return from the 1994 FIFA World Cup, where he scored an own goal, the first of a 2-1 defeat to the USA that knocked out the Colombians at the first phase. In the most believed explanation, the Medellín drug cartel bet large sums of money that Colombia would advance, and blamed the Medellín-born Escobar for the loss.

In 1994, a comprehensive point shaving scheme organized by campus bookmaker Benny Silman and involving players from the Arizona State University men's basketball team was uncovered with the assistance of Las Vegas bookmakers, who grew suspicious over repeated large wagers being made against Arizona State.
On 10 February 1999, a plot to disable the floodlights of The Valley during a Charlton-Liverpool match was discovered. Three individuals were arrested, and the scam tracked to Malaysia, where the Premiership is very popular, and bets frequent
In early 2000, Hansie Cronje, then highly-regarded captain of the South African cricket team, rocked the cricketing world with frank admissions of match-fixing. Hansie admitted to receiving more than $140,000 USD from London-based bookies to influence aspects of his team's performance. For example, he convinced Herschelle Gibbs to score less than 20 runs in a One Day International for a $15,000 USD reward. Hansie received a lifetime ban from any involvement in professional cricket but he maintained throughout his numerous trials that he never consipired to fix overall match results. He died tragically in a plane crash in 2002, leaving behind many unanswered questions and a tainted legacy.

The Italian Football Federation said in October 2000 it had found eight players guilty of match-fixing. Three were from Serie A side Atalanta and the other five played for Serie B side Pistoiese. The players were Giacomo Banchelli, Cristiano Doni and Sebastiano Siviglia (all Atalanta) and Alfredo Aglietti, Massimiliano Allegri, Daniele Amerini, Gianluca Lillo and Girolamo Bizzarri (all Pistoiese). The charges related to an Italian Cup first round tie between the two sides in Bergamo on August 20, 2000 which ended 1-1. Atalanta scored at the end of the first half and Pistoiese equalised three minutes from full time. Atalanta qualified for the second round. Snai, which organises betting on Italian football, said later it had registered suspiciously heavy betting on the result and many of the bets were for a 1-0 halftime score and a fulltime score of 1-1.

In late 2004, the game between Panioniosand Dinamo Tbilisi in the 2004-05 UEFA Cup was suspected of being fixed after British bookmakers detected an unusually high number of half-time bets for a 5-2 win for the Greek side, which was trailing 0-1. As the final result ended up being 5-2, suspicions of fixing quickly emerged, but were quickly denied by both clubs, although UEFA started an investigation.

In early 2005, the German Football Association (DFB) revealed that referee Robert Hoyzer was under investigation for suspected betting on a first-round German Cup tie between regional league side Paderborn and First Bundesliga club Hamburger SV in August 2004, and possibly fixing the match. In the match, HSV took a 2-0 lead, but Hoyzer sent off HSV striker Emile Mpenza in the first half for alleged dissent (a sending-off that many observers considered unwarranted), and later awarded Paderborn two dubious penalties. Paderborn went on to win 4-2. Several days later, Hoyzer admitted to having fixed that match, as well as several others he worked. He went on to implicate other referees and several players in the scandal. Hoyzer himself was arrested on February 12 after evidence emerged that he may have fixed more matches than he had admitted to fixing. On February 16, UEFA announced that it would send an investigator to Athens to investigate possible links between this scandal and the aforementioned Panionios-Dinamo UEFA Cup tie. Eventually, Hoyzer was sentenced to 2 years and 5 months in prison. The Croatian betting syndicate which had paid Hoyzer to fix matches was also found to be linked to the Panionios-Dinamo match.

In late September 2005, two referees (Edilson Pereira de Carvalho and Paulo Jose Danelon) were accused of fixing several matches in the São Paulo championship for an internet betting ring that moved over USD100,000 on each match day, receiving around USD 4,400 for each match [6]. In the following days, Armando Marques, president of the national commission of referees resigned and Nagib Fayad and Vanderlei Pololi, two businessmen, were arrested as suspects of working as middlemen between the referees and the corruption ring. In early October, a court ordered that the matches where Carvalho was the referee would have to be replayed and free to the public. No decision was made about Danelon's matches.
In 2006, Italy would be rocked by an even bigger betting scandal which saw four of the top teams in Serie A convicted of match-fixing charges. Juventus, which had won the league title in 2006, was stripped of its title and forcibly relegated to Serie B. Fiorentina and Lazio were initially relegated, but were restored to Serie A on appeal. AC Milan were not relegated. All four clubs were given substantial points penalties in the 2006-07 season. In a later investigation, a fifth Serie A club, Reggina, was also found guilty of match-fixing charges and was fined and given a substantial points penalty. Nearly twenty individuals were also personally punished, generally with fines and suspensions.