SPORTS play a very large part in American life. This worries some people who see sports overemphasized, but delights others for whom an arena or ballpark is paradise. Like it or not, public interest is extraordinarily high, and is growing.A survey for Sports Illustrated by Lieberman Research found that half the adult population attended at least one sports event in the past year - and averaged eight such events. It's an area where the "gender gap" is shrinking: Women were less likely to have paid to attend (39 percent) than men (57 percent), but the margin is down. As with virtually every aspect of sports including personal participation, there's a big socio-economic skewing in the attendance figures. For example, 60 percent of those with college training paid to attend in the last year, compared to just 26 percent of those with less than a high school education. Fans complain a lot about professional athletes' salaries - which average $300,000 in the National Football League (NFL), $600,000 in baseball, and $900,000 among National Basketball Association (NBA) teams - and the sharp climb in ticket prices. But this hasn't stopped ticket sales. Between 1965 and 1990 major league baseball attendance jumped 140 percent, NFL games by 170 percent, and NBA contests by an extraordinary 780 percent (2.1 to 18.6 million). In this quarter-century, the country's real gross n ational product doubled, a fact which probably more than anything else explains the surge in sports attendance. Against this backdrop of "more" virtually everywhere, the three major professional team sports have experienced drastic shifts in their fortunes. One gets different pictures depending on the measure, but football is now securely No. 1 in fan interest, having vanquished baseball as the "national pastime." Gallup first asked Americans "What is your favorite sport to watch?" back in 1937. Some 34 percent said baseball, 23 percent football, and 8 percent basketball. These numbers changed little during the 1940s, 1950s, and early 1960s. But in the next decade baseball's fortunes plummeted, while football's soared. In 1972, 36 percent told Gallup football was their favorite sport. Just 21 percent picked baseball. And 8 percent thought basketball best. Over the last 20 years, basketball has made the big move in fan interest (15 percent picking it as their favorite spectator sport in a 1990 Gallup survey), while football has maintained its roughly 35 percent share. Baseball dipped. An end to these reversals isn't in sight. The 1990 Gallup study found baseball much more popular among those 50 years and older than among the young, whereas both football and basketball do best among those 18 to 29 years of age. Baseball's relative lack of support among black Americans is shown clearly in the Gallup data: 35 percent of blacks picked basketball and 33 percent football, while only 10 percent said baseball was the sport they most liked to watch. The number of black respondents in this survey isn't big enough to permit us to judge precisely the preferences of the various age groups, but it seems clear that interest in baseball as "my sport" has virtually disappeared among young black Americans. Baseball is the one professional sport an academic or writer can claim to love without risk of having his "practicing intellectual" card revoked. But in the public at large, the college-trained like baseball the least and dote most on football. Those who haven't graduated from high school are, proportionately, baseball's biggest fans. Women give football less fan support than men - but more than baseball, by two to one. There's been endless speculation about professional football's surge in popularity in the 1960s, much of it centering around such factors as gambling (a "good betting game" compared to baseball) and technology (TV and football as the "perfect marriage"). Still, some observers are surprised by football's hold. "I would have predicted 15 years ago that we'd be seeing a clear decline in football interest," Boston Globe sports columnist Bob Ryan remarked in a recent interview with the Roper Center's magazine, The Public Perspective. "I had thought we were heading, to borrow a phrase, to a kinder, gentler era in sports." In this wealthy, sports-obsessed nation, there is plenty left over for baseball; ballparks are bulging. Still, the sport shows no signs of recapturing the special place it so long occupied.