Between 1961 and 1977, the professional sports leagues expanded. MLB added ten teams, the NFL six, the AFL two, and the NBA fourteen. In organized baseball six clubs joined the AL and four the NL. Demographically, AL teams located in metropolitan areas with smaller populations and higher per capita incomes compared to their NL counterparts. New York, Los Angeles, and Houston were the largest, and Kansas City, San Diego, and Seattle the smallest cities hosting the new franchises. The majority of teams played better before more fans during their first to tenth season. As the seasons progressed, however, the Montreal Expos disappointed their fans. Following expansion the team's five- to ten-year average attendance declined (see Table 2.4).
The areas of the expansion teams in professional football were usually below the league average in population and per capita income but above average in population growth. The largest cities selected by team owners were Minneapolis and Seattle. Except for the New Orleans Saints, the teams improved in winning percentage after expansion. Surprisingly, the average attendance of only the Dallas Cowboys and Minneapolis Vikings rose in the ten seasons following the expansion year. In 1970 the NFL expanded to twenty-six teams. Four former AFL clubs joined the AFC's Western and Eastern Divisions, and two the Central Division. From 1970 to 1974, and 1975 to 1979, five of the teams played better and drew more fans to their stadiums.
Ten of the NBA's expansion franchises existed for ten years at their original sites. The Zephyrs moved to Baltimore after two seasons in Chicago, the Rockets to Houston after four years in San Diego, the Jazz to Salt Lake City after five years in New Orleans, and the Braves to San Diego after eight seasons in Buffalo. The areas of the expansion teams fell below the league average in population and above average in population growth and per capita income. From the expansion year six teams played better and seven clubs drew more fans to their arenas. In sum, over 50 percent of teams in the three sports won more games and played before larger home crowds after the expansion year. This suggests that the leagues decision to expand into untapped markets proved to be in the best interests of the owners, fans, and the respective communities hosting the teams.