Language shift, also known as language transfer or language replacement or language assimilation, is the process whereby a speech community shifts to a different language, usually over an extended period of time. Often, languages that are perceived to be higher status stabilise or spread at the expense of other languages that are perceived by their own speakers to be lower-status. An example is the shift from Gaulish to Latin during the time of the Roman Empire. Mechanisms Prehistory For prehistory, Forster et al. (2004) and Forster and Renfrew (2011) observe that there is a correlation of language shift with intrusive male Y chromosomes but not necessarily with intrusive female mtDNA. They conclude that technological innovation (the transition from hunting-gathering to farming, or from stone to metal tools) or military prowess (as in the abduction of British women by Vikings to Iceland) causes immigration of at least some men, who are perceived to be of higher status than local men. Then, in mixed-language marriages, children would speak the "higher-status" language, yielding the language/Y-chromosome correlation seen today. Assimilation is the process whereby a speech-community becomes bilingual and gradually shifts allegiance to the second language. The rate of assimilation is the percentage of the speech-community that speaks the second language more often at home. The data are used to measure the use of a given language in the lifetime of a person, or most often across generations. When a speech-community ceases to use their original language, language death is said to occur. Indo-European migrations In the context of the Indo-European migrations, it has been noted that small groups can change a larger cultural area. Michael Witzel refers to Ehret's model "which stresses the osmosis, or a "billiard ball," or Mallory's Kulturkugel, effect of cultural transmission." According to Ehret, ethnicity and language can shift with relative ease in small societies, due to the cultural, economic and military choices made by the local population in question. The group bringing new traits may initially be small, contributing features that can be fewer in number than those of the already local culture. The emerging combined group may then initiate a recurrent, expansionist process of ethnic and language shift. David Anthony notes that the spread of the Indo-European languages probably did not happen through "chain-type folk migrations", but by the introduction of these languages by ritual and political elites, which are emulated by large groups of people. Anthony