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The competition is keen. In 2000, the average home in the United States had access to 61 channels, Nielsen says. By last year, the typical home had 104 channels.

Until the “Who shot J.R.?” phenomenon 27 years ago, broadcast networks more often than not drew little attention to the end of the season in May. Mr. Brooks says the strategy of seeking a ratings boost through season-ending special events and series cliffhangers may be counterproductive.

“It may bring you back in the fall to see what happens, but it also tells you very explicitly that there’s nothing going to happen until the fall,” he adds. “Viewers are taking that as a signal to go elsewhere.”

Broadcasters are hardly blind to what’s happening.

They pay so much for scripted programming that economic reality locks them into repeating episodes to get extra ad revenue. Ratings that may make a series a profitable hit on cable represent failure on the bigger stage of a broadcast network. Unless someone figures out something different, networks are locked into concentrating on cheap reality programming in the summer, hoping to catch lightning with an unexpected hit.

“We all tried a little bit,” says Alan Wurtzel, NBC’s chief researcher, “but there is no way you can have a full-fledged, 52-week season, both financially and creatively.”

The result is ABC, CBS, NBC and Fox sitting back for another few months as the cable competitors continue to chip away at what always has made them special. That’s another few months of viewers not checking those networks first when they settle into their couches.

The broadcast networks suffered through an unexpectedly slow spring.

Can they afford this bummer of a summer?