Heinlein also created terrific women role models in his early short stories. They included G. B. McNye, a radio engineer who coeducates an all-male space station in "Delilah and the Space Rigger," and M. L. Martin, a world-famous scientist in "Let There Be Light." Often these women used their initials instead of their full names -- Gloria Brooks and Mary Lou for instance -- a practice that, on the eve of fourth grade, inspired me to abridge Mary Grace to M. G.

When college-age women tell me they cannot imagine a world in which opportunities for women were so openly curtailed, I suggest they screen some cold-war-era classroom films, as I recently did. Among these films is the emblematic "Why Study Science?" (1955), in which a boy announces he plans to study science so he can "go to the moon," but his sister doesn't have to because her mission is to "hook some guy." "What's wrong with that?" his sister asks. The idea that you don't need science to prepare nutritious meals! their mother counters. Or to teach your toddler how the telephone works. Heinlein managed to ridicule such sexism without alienating his core audience of engineering-minded boys. In "Tunnel in the Sky" (1955), a young-adult novel, Heinlein describes a 10-day wilderness survival trip that goes awry, stranding a group of high school students on a hostile planet. Rod, the book's central character, believes that teaming with a girl will hurt him, since girls are flighty, unstable and mechanically inept. Instead, he partners with the eccentric boy who rescues him -- a boy with no facial hair who never sheds his body armor. The reader immediately grasps what Rod is too bigoted to notice: The "boy" who keeps him alive is a girl.

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Heinlein was decades ahead of his time in the way he played with preconceptions about masculinity and femininity. Far from being essential to biology, Heinlein suggests, gender is socially constructed; it involves approaching an impossible absolute by approximating it. In "The Moon Is a Harsh Mistress" (1966), he introduces a computer that develops consciousness. As a heap of circuits, it has no biological gender; but it moves easily between "Mike" and "Michelle," fully realized male and female identities. Likewise, in "The Star Beast," he creates a gigantic, steel-eating "Lummox" whom John Thomas, a human boy, keeps in his backyard as a pet. When Heinlein describes the creature noshing on a secondhand Buick, the reader pictures a big, oafish guy. As it turns out, however, Lummox is female. And far from being a brute, she is a highly cultivated empress, an emissary from an advanced alien species, who views John Thomas as her companion animal.

In many novels, Heinlein's characters commit to group marriages -- less because the arrangement offers sexual variety than because it can free women to work. It also provides efficient parenting, an argument that appealed to me as a child. When my mother died of cancer during my girlhood, I wished my father had had a bunch of other mothers around to pick up the slack.

Given Heinlein's apparently feminist ideas, you'd think he would be enshrined as a champion of women's rights. And had he stopped writing with his young-adult novels, he most likely would have been. But the sexual revolution took a toll on him, tainting some of his post-1970 novels with a dated lasciviousness and impairing his ability to create three-dimensional women. In Heinlein's earliest stories -- the ones in which lady scientists used their initials -- Heinlein eroticized his women. But the prim conventions of 1950's fiction precluded doing this explicitly. By the 1980's, however, he felt licensed to reveal more -- or, in the case of Friday, to describe sexual experiences from a woman's point of view. Friday is an "Artificial Person"; she was conceived in vitro and brought to term in an incubator, which in the book's fictive world is a terrible stigma. To today's AIDS-conscious reader, however, Friday bears a worse stigma: she is a brazen disease vector, recklessly promiscuous, with a bizarre weakness for male engineers. (Heinlein trained as an engineer.) This gives unintended meaning to the idea of Artificial Person; Friday exists only as a mouthpiece. Heinlein has so thoroughly objectified her that her subjectivity falls flat.

Sometimes I wish Heinlein were a less complex writer, that I could cheerlead for his early novels and ignore the rest. Instead he leaves me -- and his other feminist admirers -- with a paradox: How in later life could someone who once wrote so expansively produce work that only a Mother Thing could love?

ESSAY M. G. Lord is the author of "Astro Turf: The Private Life of Rocket Science" and "Forever Barbie: The Unauthorized Biography of a Real Doll."