Fighting

Fighting is the most negative social interaction in all games in The Sims series. When two Sims dislike each other enough, they can start a full on fistfight.

When fighting occurs a cartoon-styled cloud of smoke will appear and obscure most of the fighting. Sims with higher Body or athletic skill  skill are more likely to win the fight. A police officer and a burglar will fight if the police are summoned during a burglary.

The Sims
In The Sims, the cloud of smoke will completely obscure the fight.

If an adult Sim loses a fight, they may leave the neighborhood forever by saying "I'm leaving for good!". Children can also have fights, but they will never leave town if they lose.

In The Urbz: Sims in the City, Fight can be chosen regardless of relationship.

As the Fight social type did not exist in The Sims and The Urbz: Sims in the City, Attack/Fight was a mean social.

The Sims 2 and The Sims Stories
Fights in The Sims 2 and The Sims Stories are a little more graphic and violent. Sims will pop up occasionally through the cloud of smoke, and will use a small variety of professional and comical fighting techniques. A Sim who is a child or older can start and watch fights. Sims with a higher body skill are more likely to win fights. When a fight breaks out, nearby Sims' reactions to the fight will be based on their personality. Mean Sims will taunt the fighters, nice Sims will watch distressed, and shy Sims will run and hide. If the Sim is a parent of one of the fighters, they will watch distressed, even if they are mean; likewise, if the Sim is a child of one of the fighters, they will always run from the room, usually to their bed, and cry or throw a tantrum. Selectable Sims in the area of the fight will have the "Watch Fight" interaction pushed on them. The player can cancel this, but it may be pushed on the Sim again. If one Sim cheers for a certain Sim in a fight, he/she is more likely to win. Sims with 6 or more nice points will always watch distressed even if they have a good relationship with one of the fighters. Beginning with Nightlife, a fight can trigger fury. This can also happen in The Sims Stories. If a Sim catches their children fighting, they will lecture the child. After a fight, the Sims involved get a memory of winning or losing a fight. In The Sims 2 and The Sims Stories, Attack is a Fight social instead of a Mean social. The chances of winning the fight is modified by approximately +5% for each body point the Sim has that the other Sim doesn't, or vice versa.

If Pets is installed, two pets can start a fight with each other if their relationship is low enough. Like normal Sim fights, nearby Sims will watch the fight and react differently depending on their personality.

The Sims 3
In The Sims 3, Sims can initiate a fight if they're very angry at each other. A Sim with the mean spirited trait has the "Beat Up" interaction, while a Sim with the proper trait has the "Give (Sim name) What For" interaction, on which the Sim will harass the other Sim before fighting. Sims with the proper trait cannot properly initiate fights, or attempt to defend against burglars even if they have the brave trait. Sims with the good trait cannot initiate fights at all, but they can still get involved in them, as well as win.

The outcome of a fight is based on several factors. Some factors cause a Sim to instantly win regardless of any other factors. The outcome is determined by the following, in that order:
 * 1) If either of them, not both, has a skill in martial arts, the Sim with the skill will win regardless of any other factors.
 * 2) If either of them, not both, has the loser trait, the Sim with the trait will lose regardless of the following factors.
 * 3) If either of them, not both, has completed the Body Builder athletic skill challenge, the Sim with the achievement will win regardless of the following factors.
 * 4) The Sim who initiates the fight has a 50% chance to win.
 * 5) The chance is modified by +8% per each level of difference in Athletic skill. The chance is negative if the opponent has higher Athletic skill.
 * 6) If the Sim has either Brave, Evil, or Mean Spirited trait, or "Oddly Powerful" moodlet, the chance is modified by +40%. The trait/moodlet benefit doesn't stack. The chance is negative if the opponent has the trait. So if both Sims have the Brave trait, the modifier becomes +0%.
 * 7) If the Sim has the Can Apprehend Burglar hidden trait, the chance is modified +25%. This benefit is only applied to the initiator, so it has no effect if the opponent has this hidden trait.
 * 8) After that, the game will determine the winner based on the chance. If the chance exceeds 100%, the initiator is guaranteed to win. If it's below 0%, the initiator is guaranteed to lose.

The winner will receive the "Winner" moodlet, while the loser will receive "Humiliated" moodlet. A celebrity who engages in a fight may be publicly disgraced. SimBots and plumbots will always win against mummies. Bonehilda always wins against anyone.

Brawling
In The Sims 3: Late Night, Sims can brawl in addition to fight in a bar, club, or lounge (except future lounge). Brawling has a completely different system from fighting. A Sim can initiate a brawl with another, and more Sims can join (Sims may only autonomously join a brawl in a bar lot assignment). A brawl may last for 15–25 minutes, and additional 7–10 minutes per Sim involved. During a brawl, various debris will fall out of the fight cloud in a comical way.

When the brawl is over, the game will decide if it becomes a friendly brawl or not. If the average relationships between the participants reach 30, they will all earn +10 relationship boost and a "Great Brawl" moodlet. Otherwise, the game will determine the winners and losers. The game will calculate the brawl score of each Sim, and the top half of the participants will be the winners, while the others become the losers. Winners will earn +8 relationship boost with other winners, a "Won the Brawl" moodlet, and -6 relationship drop with all the losers, while losers will get a "Lost the Brawl" moodlet.

The brawl score is calculated as following:
 * +1 is earned for each skill point in Athletic and Martial Arts.
 * +5 is earned for each of the following trait a Sim possesses: Athletic, Hot-Headed, Lucky.
 * -8 is earned for each of the following trait a Sim possesses: Loser, Unlucky.
 * +4 is earned for each of the following moodlet a Sim possesses: "Oddly Powerful", "Boosted Courage", "Vampiric Vigor".
 * -4 is earned for each of the following moodlet a Sim possesses: "Sore", "Exhausted", "Starving".
 * +20 is earned if the Sim is an occult.
 * The relationship score between a Sim and each of the participants times 0.25 is added to the brawl score.

If more than 7 Sims are involved in a brawl, police will come to stop and lecture the Sims. There's a 25% chance a Sim will get either "Backache", "Sore", or "Hurt Hand" moodlet after brawling.

The Sims 4
It is same as The Sims 3, possibly.

Trivia

 * Young adults, adults and elders may fight with each other. In The Sims 2, children may also start fights, but not with older Sims. In The Sims 3, children may not start fights with young adults or older, but they may fight with teens, and may even win such fights.
 * In some versions of The Sims 2 (confirmed with just Nightlife and Pets), if a toddler witnesses an affair happen and is furious with an adult because of it, the adult may initiate a fight with them as well. No animations play for either character, although the fight clouds will still appear.
 * In The Sims 3, some Sims will laugh at the loser if a fight breaks out, and Sims with the evil trait will point at the loser and cackle. In addition, grumpy or hot-headed Sims will break out into a rage if a fight starts.
 * In The Sims 3, a common misconception is that having high martial arts skill will increase the chance of winning a fight. In fact, it doesn't, and it only matters whether the Sim has a skill in martial arts or not.
 * In The Sims 3, another common misconception is that some traits are considered to affect the outcome of a fight, while they're not. These traits are, for example, athletic, lucky, hot-headed, daredevil, and unlucky.
 * In The Sims 3: Supernatural, a werewolf will get "Top Dog" moodlet after winning a fight or brawl.
 * In The Sims 3: Late Night, a celebrity who gets into a fight will get the "Publicly Disgraced" moodlet.
 * In The Sims 4, Sims who are beaten up will become dazed. However, adult Sims get angry after witnessing a child fight.