Rock music is a genre of popular music that developed during and after the 1960s, particularly in the United Kingdom and the United States. It has its roots in 1940s and 1950s rock and roll, itself heavily influenced by rhythm and blues and country music. Rock music also drew strongly on a number of other genres such as blues and folk, and incorporated influences from jazz, classical and other musical sources. Musically, rock has centred around the electric guitar, usually as part of a rock group with bass guitar and drums. Typically, rock is song-based music with a 4/4 beat utilising a verse-chorus form, but the genre has become extremely diverse and common musical characteristics are difficult to define. Like pop music, lyrics often stress romantic love but also address a wide variety of other themes that are frequently social or political in emphasis. The dominance of rock by white, male musicians has been seen as one of the key factors shaping the themes explored in rock music. Rock places a higher degree of emphasis on musicianship, live performance, and an ideology of authenticity than pop music. By the late 1960s a number of distinct rock music sub-genres had emerged, including hybrids like folk rock, blues-rock, country rock and jazz-rock fusion, many of which contributed to the development of psychedelic rock influenced by the counter-cultural psychedelic scene. New genres that emerged from this scene included progressive rock, which extended the artistic elements; glam rock, which highlighted showmanship and visual style, and the diverse and enduring major sub-genre of heavy metal, which emphasized volume, power and speed. In the second half of the 1970s, punk rock both intensified and reacted against some of these trends to produce a raw, energetic form of music chraacterised by overt political and social critiques. Punk was an influence into the 1980s on the subsequent development of other sub-genres, including New Wave, post punk and eventually the alternative rock movement. From the 1990s alternative rock began to dominate rock music and break through into the mainstream in the form of grunge, Britpop, and indie rock. Further fusion sub-genres have since emerged, including pop punk, rap rock, and rap metal, as well as conscious attempts to revisit rock's history, including the garage rock/post punk revival at the beginning of the new millennium. Rock music has also embodied and served as the vehicle for cultural and social movements, leading to major sub-cultures including mods and rockers in the UK and the "hippie" counterculture that spread out from San Francisco in the US in the 1960s. Similarly, 1970s punk culture spawned the visually distinctive goth and emo subcultures. Inheriting the folk tradition of the protest song, rock music has been associated with political activism as well as changes in social attitudes to race, sex and drug use, and is often seen as an expression of youth revolt against adult consumerism and conformity.