The amount of care given to a Tamagotchi will affect its growth. Different characters are obtained through different requirements. These requirements are always described on charts showing each growth, requirement and character.
Involvements and Guidelines
Hunger and happiness
Hunger and Happiness determine how hungry or happy a Tamagotchi is, normally indicated by four or five hearts. On color Tamagotchi releases, hungry hearts are replaced with rice bowls and the happiness hearts is represented as a gauge unit which becomes longer each time a TamaTomo stamp is earned.
Hunger is filled each time a meal is fed. Happiness are filled in multiple ways, such as feeding a snack or playing a game. There are two "hidden" hearts on the meter, which means that a Tamagotchi can be fed another two meals after its hungry hearts are all filled. It's the same with happiness, except that snacks can still be fed even after the two hidden happiness hearts are filled. At child or teen stage, feeding a Tamagotchi approximately fifteen snacks in succession will lead to it getting a cavity, however.
In the FamiTama and Version 5, as adults, missing three consecutive calls for empty hungry hearts will result in an evolution to Puchiputchi. Missing three consecutive calls for empty happiness hearts will result in evolutions of Gozarutchi and Kunoitchi. Overfeeding on three occasions by filling up seven hearts will result in an evolution to Osumotchi. Likewise, In the Royal FamiTama and Version 5 Celebrity, as adults, missing three consecutive calls for empty hungry hearts will result in an evolution to Modeltchi, missing three consecutive calls for empty happiness hearts will result in evolutions to Paparatchi, and overfeeding them on three occasions by filling up seven hearts will result in an evolution to Puddingtchi.
In the Tamagotchi Angel, the Happiness meter is referred to as the "Effort meter".
Stress
Stress is a feature exclusive to Tamagotchi Music Star. If a Tamagotchi's stress level is kept low, it will become happier and will evolve into a character of better care. The highest stress level is 99. If a Tamagotchi becomes too distressed it will eventually skip concerts, refuse to practice, and so on. Eating food besides Baby Food and Baby's Milk, playing games besides the ones accessible from the "Away" menu, and playing with its favorite toy, as well as praising it, will lower a Tamagotchi's stress level. However, practicing, being given a time out or a toy it doesn't like, the games accessible from the "Away" menu and becoming sick will all increase stress.
Training
In the vintage era, a Tamagotchi might call out for no reason. When this happened, users had to select the discipline icon, therefore adding to the training meter. In the Tamagotchi Angel, the equivalent to discipline is praise, used whenever the angel praying. Praising a praying angel will increase their good deeds meter. If praising whenever it was not the case, one happiness/effort heart will be deduced. A similar concept was introduced with the Genjintch Tamagotchi except making pottery and praising would increase the evolution meter, which would contribute to evolution. In the TamaOtch, both a training and a discipline meter exist. The training meter is different from discipline, and in this version alone, refers to how much training (the forth icon from left on the first row) has been completed.
The training feature is the same, but is now split into "Time Out/Punish" (しかる scold) and "Praise" (なでる praise).
Weight
Weight is a factor appearing in all releases. Weight increases when a Tamagotchi is fed and decreases when games are played. This feature was replaced with "Tenshitch Power" on the Tamagotchi Angel and is similar to the Devilgotchi's "Debirutch Power" feature.
Care Mistakes
A Care Mistake, also known as a 'care miss', a 'miss care', or simply a 'miss', is a variable that appears on all Tamagotchi releases. It is a measure of how many times the user has forgotten to care of their Tamagotchi, which factors into what character it will evolve into or how close it is to Death. While it is not possible to measure the total number of Care Mistakes at any given time, there will be an end result of them.
When one or more meters empty, the "Attention" icon appears, accompanied by an audible signal, and in the modern Tamagotchi releases, a speech bubble which contains either the user's name (or what the user entered for their name) or a phrase such as "I'm hungry", or a flashing exclamation mark above the character's head. If fifteen minutes have elapsed and the user has not tended to the Tamagotchi's needs, the attention icon will turn off and the additional graphic will disappear, resulting in a care mistake. If both meters empty and are not tended to before the attention icon turns off, this counts for two care mistakes. Not turning the lights off after a Tamagotchi has gone to bed will result in a care mistake as well.
Failure to respond to a call within fifteen minutes will result in it becoming sick. This can also happen if poop is left on the screen for a certain amount of time, or if it is untreated when sick. Missing calls for discipline will not result in Care Mistakes. A certain number of Care Mistakes are required to obtain a specific character on most releases; for example, on the Tamagotchi iD L 15th Anniversary Version, Young Oyajitchi can evolve into Oyajitchi with at least 6 care mistakes.
If more care mistakes continue to occur, it may either die or attempt to run away from home. Some modern Tamagotchi releases, such as the Keitai Kaitsuu Tamagotchi Plus, Chou Jinsei Enjoi Tamagotchi Plus and the Tamagotchi Plus Color, have ways of preventing death; should death be avoided, one Care Mistake will be reversed. Care Mistakes can also be reversed through the use of a special medicine obtainable on some versions, although it has no effect on evolutions.
Friendship
Friendship is a feature exclusive to the TamaTown Tama-Go. The more hearts filled, the better impact it will have on the Tamagotchi's growth.
Vintage History
Care-based evolution has been retained across all Tamagotchi releases from 1996 to date. The vintage era created a system involving a certain teen having a certain set of possible adulthood evolutions. In the original Tamagotchi, the better care teen would be able to evolve into all six possible adults, whilst the worse care teen could only become three of them.
Modern History
In Tamagotchi Connection Version 2 and Version 3, the care given to a parent determines the child evolution which will occur from the resulting baby. If the parent was given good enough care when a teen, the baby could develop into Kuribotchi or Kinakomotchi on the V2, and Kuchitamatchi or Mohitamatchi on the V3.
Care does not affect the growth in the Tamagotchi Connection Version 5, the Japanese FamiTama and their Celebrity-themed remakes. Instead, there is a new feature called Bonding, which steers the evolution in these versions. That said, however, there are three neglect-exclusive families obtainable at adult stage.
A similar system to that of the original Tamagotchi exists for the Tamagotchi Plus Color, Tamagotchi iD, iD L, and the P's, although they introduced a new feature known as care mistakes.
On the TamaTown Tama-Go, when the character is an adult it can evolve into Otokitchi or Ojitchi through neglect. It can evolve into an oldie at a young age if given enough neglect.
The Tamagotchi 4U's Post-Adult form involved a certain adult character evolving into characters revolving around that character. The method of obtaining these characters depends on the original character. For instance, if Mametchi is cared for well he can evolve into Smart Mametchi and if he is neglected he can evolve into Naughty Mametchi.