Brothers & sisters

Then there was “The Brady Bunch.’’

A guilty pleasure as sweet and light as cotton candy, “The Brady Bunch’’ debuted in the final week of September 1969. Forty years later - long after its ambitious competition has faded into obscurity - the show that never cracked the Nielsen top 25 during its original five-year prime-time run is still seen daily in syndication around the world and has become an inescapable part of pop culture. “The Brady Bunch’’ is adored as much as it is maligned, and as the show reaches middle age, it is the fictional family that America refuses to forget.

“In its post-network life, ‘The Brady Bunch’ has been something that we totally embrace and totally mock’’ says Syracuse University pop culture professor Robert Thompson. “I think ‘The Brady Bunch’ was one of the first shows that became reconceptualized in people’s minds in the 1980s and 1990s with that tongue-in-cheek attitude. ‘The Brady Bunch’ became the first classic cheesy show where we really started to embrace the cheese.’’

Few will deny that, at times, the Bradys can be cheesier than a plate of nachos - the polyester blouses and elephantine bell bottoms alone make the show entertaining. But laughing at bad fashion doesn’t explain the enduring popularity or the string of reunion shows, cinematic reinventions, and the growing number of Brady books that Americans happily consume. The blended family of widower Mike Brady and his three sons, along with his quietly divorced wife, Carol Martin, and her three blond daughters have, like it or not, become a part of everyone’s extended family.

Christopher Knight, who played Peter Brady, the middle son, offers another explanation as to why the show has stuck and refuses to dislodge itself from our collective consciousness.

“There seems to be a natural tendency for kids to be drawn to the show,’’ says Knight, 51, who now hosts the game show “Trivial Pursuit.’’ “It became something kids either identified with, or it became their surrogate family. Its simplicity is perhaps why it succeeded. It was perfect for children, and that’s what this was, a show for children.’’

“The show also didn’t date itself with anything socially relevant,’’ he continues. “Much to our frustration at the time that we were doing it. That left the show capable of being evergreen. The only thing really dating it is the architecture, the interior design, and the clothing. Because ultimately family structure remains, for the most part, the same.’’

A shared, multigenerational history of “The Brady Bunch’’ has also helped it exist well beyond that initial five-year run. Shortly after the show exited prime time, it went into syndication - and stayed there. Before most families had cable or VCRs, the afterschool program of choice was “The Brady Bunch,’’ and it was watched by millions of kids through the 1970s and 1980s. It then made the rounds on TBS and Nick at Nite for yet another generation before landing on TV Land.

“They’re shows that you can watch over and over again,’’ says Jaclyn Cohen, senior vice president of programming and acquisitions for TV Land. “You want to go with them to Hawaii and the Grand Canyon. You want to see Marcia get hit in the nose with the football or hear Cindy’s secret.’’

Eve Plumb, who played middle sister Jan Brady, has a slightly more pragmatic view of why the show has persisted. (Full disclosure: Plumb is a friend of this writer.) She thinks part of its longevity is that it is inexpensive to rebroadcast. Actors stopped receiving residual checks from the show long ago.

“It’s so well known because they’ve managed to keep it on the air,’’ says Plumb, 51, who now devotes her time to painting. “It’s easy to like what’s on all the time. It’s the way I am with episodes of ‘Law & Order.’ I’ve seen them all - twice.’’

Like Knight, Plumb says the fact that the show dispensed simple lessons about right and wrong and was squeaky clean - save for the time Greg took up smoking - also makes it a good baby-sitter. Moms are still popping kids in front of the show because, as Plumb wickedly adds: “They know there was never an episode where Sam the butcher beheads Alice.’’

Faith Soloway, a Boston-area performer who staged a popular show called “The Real Live Brady Bunch’’ in the early 1990s, contends that the show had an almost magical effect on her and her sister.

“I don’t think the show intended to brainwash us, but it did,’’ she says. “It’s really the perfect family that we all wanted to be a part of. Except for maybe the divorce that was never talked about. Or the dead mother who was never talked about.’’

Susan Olsen, better known as Cindy Brady, is the latest cast member to write a book on the Bradys. “Love to Love You Bradys’’ looks at the fictional clan’s disastrous 1977 variety show. It is affection for her costars more than the material that has brought the cast together for several Brady reunions.

“We missed each other,’’ she says, explaining why the cast agreed to the tacky variety show. “I think the idea of working together again was appealing.’’

Olsen sees the show’s lasting appeal as the result of two specific factors. First, she says, the episodes always focus on the childrens’ problems, not those of the parents. Everyone is concerned when Cindy loses her doll or Greg hides a goat in his bedroom. The other reason has to do with the actors behind the characters.

“I think in order to have eternal life, you have to have a soul,’’ she says. “And the soul of the show is that we all really did love each other. I think that really came across.’’

Christopher Muther can be reached at muther@globe.com.