The success of the longest-running sitcoms can be attributed to their timelessness and adaptability; they have to survive new decades, after all. In some ways, situational comedies have evolved quite a bit over the years; in other ways, many still center on a family and their lives, or on employees and their jobs. The format was popular among families in the mid-20th century, as television became a medium to propagate nuclear family values in America. Though sitcoms have retained their significance, their characters, themes, and overall presentation have evolved drastically.