In 1975 Robin Lakoff wrote a book entitled Language and Woman's Place. In a related article called "Woman's Language," she talked about tactics women often use in conversation that men do not use as often, such as saying "kind of" or being overly polite. This implied that women are sociologically inclined to use insecure or less authoritative speech.
In the same year, a study was done by Don Zimmerman and Candace West UC Santa Barbara. This study was the inspiration for "dominance theory," and concerned the number of interruptions in conversation clips using both men and women. The high number of male interruptions and low number of female interruptions implied that men "dominate" conversation.
"Difference theory" came from a study by Deborah Tannen in which she suggested that women and men prioritize contrasting values, instead of the idea that men have stronger values than women. For instance, men prioritize independence while women prioritize intimacy.
A study by Janet Holmes in 1992 showed women using tag questions because they wanted to maintain a discussion, rather than because they were uncertain.
A study by Betty Dubois and Isobel Crouch in 1975 showed that men used even more tag questions, but this study never suggested that men were less confident because of this.
In reaction to Lakoff's article and book, William O'Barr and Bowman Atkins conducted a study in 1980 on language within a courtroom. They connected Lakoff's characteristics of female speech to those used by lower-class men as well. They determined that gender was not the deciding factor, but instead it was power.
Finally, another related study was conducted by Koenraad Kuiper (1991). Kuiper studied only males, only rugby players on one team. He determined that men use insults to create a team feeling of solidarity. This was intended to contrast with the idea that women use politeness and welcoming behavior to create solidarity.