Heavy metal (or simply metal) is a genre of rock music that developed in the late 1960s and early 1970s, largely in the United Kingdom and United States. With roots in blues rock, psychedelic rock and acid rock, heavy metal bands developed a thick, monumental sound characterized by distorted guitars, extended guitar solos, emphatic beats and loudness. In 1968, three of the genre's most famous pioneers – Led Zeppelin, Black Sabbath and Deep Purple – were founded. Though they came to attract wide audiences, they were often derided by critics. Several American bands modified heavy metal into more accessible forms during the 1970s: the raw, sleazy sound and shock rock of Alice Cooper and Kiss; the blues-rooted rock of Aerosmith; and the flashy guitar leads and party rock of Van Halen. During the mid-1970s, Judas Priest helped spur the genre's evolution by discarding much of its blues influence, while Motörhead introduced a punk rock sensibility and an increasing emphasis on speed. Beginning in the late 1970s, bands in the new wave of British heavy metal such as Iron Maiden and Saxon followed in a similar vein. By the end of the decade, heavy metal fans became known as "metalheads" or "headbangers". The lyrics of some metal genres became associated with aggression and machismo, an issue that has at times led to accusations of misogyny. During the 1980s, glam metal became popular with groups such as Bon Jovi, Mötley Crüe and Poison. Meanwhile, however, underground scenes produced an array of more aggressive styles: thrash metal broke into the mainstream with bands such as Metallica, Slayer, Megadeth and Anthrax, while other extreme subgenres such as death metal and black metal became – and remain – subcultural phenomena. Since the mid-1990s, popular styles have expanded the definition of the genre. These include groove metal and nu metal, the latter of which often incorporates elements of grunge and hip-hop. Heavy metal is traditionally characterized by loud distorted guitars, emphatic rhythms, dense bass-and-drum sound and vigorous vocals.