With the big mass retailers like WalMart now accounting for 40 percent or more of sales for books and music, their influence on what gets sold is growing. “But with the chains’ power has come criticism from authors, musicians and civil liberties groups who argue that the stores are in effect censoring and homogenizing popular culture. The discounters and price clubs typically carry an assortment of fewer than 2,000 books, videos and albums, and they are far more ruthless than specialized stores about returning goods if they fail to meet a minimum threshold of weekly sales. What is more, the chains’ buyers ? especially at Wal-Mart ? carefully screen content to avoid selling material likely to offend their conservative customers.”