Beginning with University, players were able to merge two households. Beginning with The Sims 2, Sims could move out on their own.
In all games, Sims can be added to a household by birth, adoption, or marriage/moving in, and can be removed by death or marrying into/moving into another household.
NPCs are excluded from the household, though some NPCs ( butlers in The Sims 3: Late Night, roommates in The Sims 2: Apartment Life, etc.) can stay there permanently. 'Family', which was the more common term in earlier games, is still used for households in which all Sims are related. The term became more common in The Sims 3, probably because Create a Sim in The Sims 3 lets players specify each Sim's last name. In The Sims 2: University, the term was used for young adults living in college sub-neighborhoods, to differentiate them from families living in other areas. A household is one group of playable Sims living on the same lot.
