You will learn a lot about formal and informal Japanese language throughout this book. In Japanese society a distinction is made between the 'in-group' (myself, my family, my work colleagues, my friends) and the 'out-group' (my seniors, people from other families and workplaces). Politeness is an important traditional aspect of Japanese life and this is reflected strongly in the language used when speaking to those in the out-group. More informal language is generally used within the in-group.
Source: wiktionary
And because people derive personal esteem and meaning from their group memberships, they are naturally inclined to evaluate their ingroups positively in comparison to outgroups. When an ingroup’s dominant social position is secure or not contested by lower-status outgroups, intergroup harmony is expected to prevail. But when perceived as illegitimate and/or challenged by other groups, ingroup members can be expected to defend their group’s social position by engaging in pro-ingroup and/or anti-outgroup behaviors.
Source: wiktionary
A cladistic analysis of 45 ingroup and two outgroup taxa was performed on the basis of 134 characters.
Source: wiktionary