Heavy metal (often referred to simply as metal) is a genre of rock music[1] that developed in the late 1960s and early 1970s, largely in the United Kingdom and the United States.[2] With roots in blues rock and psychedelic rock, the bands that created heavy metal developed a thick, massive sound, characterized by highly amplified distortion, extended guitar solos, emphatic beats, and overall loudness. Heavy metal lyrics and performance styles are generally associated with masculinity and machismo.[3] The first heavy metal bands such as Led Zeppelin, Black Sabbath and Deep Purple attracted large audiences, though they were often critically reviled, a status common throughout the history of the genre. In the mid-1970s Judas Priest helped spur the genre's evolution by discarding much of its blues influence; Motörhead introduced a punk rock sensibility and an increasing emphasis on speed. Bands in the New Wave of British Heavy Metal such as Iron Maiden followed in a similar vein. Before the end of the decade, heavy metal had attracted a worldwide following of fans known as "metalheads" or "headbangers". In the 1980s, glam metal became a major commercial force with groups like Mötley Crüe and Poison. Underground scenes produced an array of more extreme, aggressive styles: thrash metal broke into the mainstream with bands such as Metallica, Megadeth, Slayer, and Anthrax, while other styles like death metal and black metal remain subcultural phenomena. Since the mid-1990s, popular styles such as nu metal, which often incorporates elements of grunge and hip hop; and metalcore, which blends extreme metal with hardcore punk, have further expanded the definition of the genre.