Gacha games are video games that use the "gacha" ("capsule toy") mechanic, which is similar to loot boxes, to induce players to spend money. Most of these games are free-to-play mobile games. In Gacha games, players spend virtual currency, which can be from a machine.
The Gacha game model began to be widely used in the early 2010s, faring particularly well in Japan. Almost all of the highest-grossing mobile games in Japan use it, and it has become an integral part of Japanese mobile game culture. Outside Japan, the game type is also gaining popularity. Examples of Gacha games are Final Fantasy: Brave Exvius, Fire Emblem Heroes, Puzzle & Dragons, Dragon Collection, Granblue Fantasy, Girls' Frontline, Monster Strike, Fate/Grand Order, and Kingdom Hearts Union X.
In these games, there are usually numerous characters, cards, or other items that players can obtain, and most of them are only obtainable via the "gacha" mechanism. The "gacha" mechanism would allow players to "spin" the gacha using a specific amount of in-game currency, which would give player a randomized character or item. Sometimes, these gacha would be limited, such that those characters and items can only be obtained within a specific event time frame. Because some of these items or characters would be given less chance to appear, typically players would need to spin the gacha many times before they can get the most desired outcome.
In these games, gacha could be essential for players to make progress in the game. Players may be given free or discounted gachas, but have to pay to get more. These games may also feature different tiers of gacha pulls, which give different sets of rewards.
The model of gacha has been compared to that of collectible trading card games as well as gambling
Types of gacha
"Complete gacha" (コンプリートガチャ), also shortened as "kompu gacha" or "compu gacha" (コンプガチャ), was a monetization model popular in Japanese mobile phone video games until 2012, when it was rendered illegal via legal opinion. Under complete gacha rules, players attempt to "complete" a set of common items in a particular loot pool in order to combine them into a rarer item. The first few items in a set can be rapidly acquired but as the number of missing items decreases it becomes increasingly unlikely that redeeming a loot box will complete the set. This is particularly true if there are a large number of common items in the game, since eventually one single, specific item is required.
Box gacha is a virtual box of set items with known probabilities. Its popularity grew around the time that the kompu gacha controversy was becoming publicized. Box gacha is generally considered more fair because as items are pulled from the box, the likelihood of receiving the desired item increases, as there are fewer items in the box. It is also possible to pull every item in the box, provided the player is willing to spend enough. For this reason, some players will calculate how much money it would take to ensure the pull the item of their choosing.
Sugoroku gacha is similar to that of a board game. Gacha mechanisms are used for the player to progress across the board, with further progress entailing better rewards. Each time that the player rolls the gacha, their character will either roll a dice or spin a wheel to determine how far the character will go.
Redraw gacha allows the player to "re-roll" the gacha if they receive an unfavorable result. Some games offer this feature for free.
Consecutive gacha improves the chances of receiving rare rewards when the player spends in bulk. As opposed to spending a set amount for individual rolls, a player can spend a larger amount in order to roll several times in a row for a slightly discounted price. At the end of the roll, the player receives all the items at once.
With step-up gacha, the player's chances of pulling a rare item is increased each time they roll. This gacha is very popular with heavy spenders, because with every roll the stakes feel higher.
Gacha that shows the exact probability of pulling rare items.
Discounted gacha usually involve special campaigns or events by the game company to allow users to roll for a lower price.
Game developers have praised gacha as being a great free-to-play monetisation strategy.   Most developers that work primarily with free-to-play games recommend it be incorporated into the game starting with the concept for maximum monetisation potential. 
It has been debated what makes gacha so addictive to so many players. Some believe that gacha games play on the inherent hunter-gatherer instinct that people have to collect items, as well as the desire to complete a set.  Others believe it is simply the replication of the thrill of gambling that brings players back time and time again. 
Criticism and controversy
In May 2012, an article was published in a conservative Japanese newspaper, the Yomiuri Shimbun, that criticized social networking games and specifically gacha for exploiting the naivety of children to make a profit. The main complaint of the article was that the gacha model too closely resembled gambling. The paper called for an investigation by Japan's Consumer Affairs Agency to prevent abuse of the system. Shortly after, the suggested investigation was performed and the model of complete gacha was declared illegal by the Consumer Affairs Agency, citing the Law for Preventing Unjustifiable Extras or Unexpected Benefit and Misleading Representation (不当景品類及び不当表示防止法),  This was done not by introducing any new legislation, but by issuing a legal opinion that virtual items could be considered "prizes" under existing legislation written in 1977 to prevent the complete gacha practice in the context of baseball trading cards. Within a month of the opinion being issued, all major Japanese game publishers had removed complete gacha rules from their games, though many developers found ways around these rules. Japanese mobile game developers, including GREE and DeNA, worked to establish a self-regulating industry group, the Japan Social Game Association, which was an attempt to coerce developers from these models, but it did not prove successful, and the Association was disbanded by 2015.
The mechanism has come under scrutiny for its similarity to gambling, and some countries require drop rates to be made public, or have banned certain practices (e.g., complete gacha). Many players also feel regret after making purchases in these games according to a survey. This type of game has also come under criticism for luring players into spending thousands of dollars at a time to get what they want, and the way gacha outcomes are presented within the game have also been criticized.
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