Portions of "Sgt. Pepper" reflect the Beatles' general immersion in the blues, Motown and other American popular musical traditions. The author Ian MacDonald writes that when reviewing their rivals' recent work in late 1966, the Beatles identified the most significant LP as the Beach Boys' "Pet Sounds", which Brian Wilson, the band's leader, had created in response to the Beatles' "Rubber Soul". McCartney was highly impressed with the "harmonic structures" and choice of instruments used on "Pet Sounds", and said that these elements encouraged him to think the Beatles could "get further out" than the Beach Boys had. He identified "Pet Sounds" as his main musical inspiration for "Sgt. Pepper", adding that " nicked a few ideas", although he felt it lacked the avant-garde quality he was seeking. "Freak Out!" by the Mothers of Invention has also been cited as having influenced "Sgt. Pepper". According to the biographer Philip Norman, during the recording sessions McCartney repeatedly stated: "This is our "Freak Out!"" The music journalist Chet Flippo stated that McCartney was inspired to record a concept album after hearing "Freak Out!"