Have viewers traditionally turned their televisions off during the summer because of the quality of programming, or have the nets purposely held back the good stuff during these months because viewers aren’t there?

This chicken-and-egg conundrum presents a tough question, but we might get closer to an answer this summer as Fox, NBC and the WB invest more heavily in firstrun programming to stem annual ratings declines in the warm-weather months.

Programming the summer poses unique challenges.

The available audience in the summer is about 14% lower than during the traditional September-May season, and 20% lower than peak winter months such as January and February, according to Nielsen data for the past three calendar years.

For networks, knowing whom to target in the summer — and what they’re inclined to watch — is key.

In general, viewers over 55 are least likely to stop watching in primetime during the summer — in fact, they’re by far the easiest segment of the aud to reach at any hour of any month.

Over the last three Augusts, for example, use of television in primetime by over-55s was only 9% lower than it was in May.

These auds generally prefer procedural dramas, newsmags and movies, and tend to shy away from most reality fare.

On the other hand, teens and young adults in the 12-34 demo are more inclined to take a vacation from TV — presumably to go to the mall, the beach or the megaplex. They watch 17% less in August than May.

“From Memorial Day on, young-adult viewership declines and doesn’t really come back until mid-August,” says Larry Hyams, VP of research at ABC. “Labor Day weekend is really a signal that summer is over, and kids and families are getting back to normal patterns.”

Hyams points out that while it’s tough to grab a large chunk of the desirable young-adult demo during the summer, shows with broad appeal are allowed to slowly build an aud.

“There’s clearly an appetite out there for something fun and different, and it helps that there’s less clutter. There’s definitely a successful history of summer launches in recent years.”

Examples of that success include “Who Wants to Be a Millionaire,” “Survivor,” “American Idol” and most recently “The O.C.”

Another dynamic of summer viewing is that auds come to the set later in the evening — and this starts as early as April with the switch to Daylight Saving Time.

In the winter months, viewership among the 18-49 demo is roughly 9% lower at 8 o’clock than 9 o’clock, but in summer the later hour produces 15% more viewers.

And there’s also something going on in the 10 o’clock hour, where viewership in the summer is about the same as in the heavily trafficked 9 p.m. slot. This is in contrast to winter, where there’s about an 8% drop-off in viewership at 10.

The younger-skewing nets have a clear disadvantage here, as Fox, UPN and the WB program only until 10 and can’t take advantage of their core auds’ summer-viewing habits.

As a group, TV analysts seem to support the broadcast nets’ resolve to get more aggressive in the summer.

“There are still many viewers who choose television on a nightly basis (in the summer), and as importantly, as many advertisers,” says John Rash, a media buyer for Campbell Mithun. “So it’s wise for the nets to think of a 52-week season because the current architecture is an artificial construct of the TV industry and not consumer behavior.”