Science generally succeeds in bringing some order to human existence — except when it does just the reverse, imposing a structure that never quite fits properly no matter how much it is tweaked. Then it just accentuates the underlying chaos.

The much-disputed, oft-revised manual of psychiatric diagnosis might serve as one illustration of this phenomenon; given that it runs to almost 1,000 pages, Hanne Blank gets a pat on the back for dispatching the equally murky entity of heterosexuality in fewer than 200, plus back matter.

One can almost hear a chorus of experts in the many sciences of sex and gender muttering that her amusing, readable synthesis is a featherweight effort, simplistic and derivative. But for those not in the field but still in the game, as it were — readers never previously moved to reason from first principles exactly what it means to be a heterosexual or act like one — Ms. Blank darts from one intriguing, thought-provoking point to another.

She is a self-described “independent scholar” in Baltimore with several volumes of erotica and a well-received history of the virgin to her credit. Is Ms. Blank herself a heterosexual? That question prompts the first of her looping mind games.

She has had romantic relationships with women in the past — so, no, right? Now, though, she is in a stable, long-term romantic partnership with a man (so, yes, right?). But her partner has a complicated genome, with some ordinary male XY cells and some that have an XXY pattern, giving him a softer, more stereotypically feminine aspect than usual, despite standard-issue male genitalia. And suddenly that word, “heterosexual,” becomes less than the helpful, scientifically precise term one might wish for.

In fact, it was coined in Germany only in the second half of the 19th century and was first used in English several decades later with the classical sense of “hetero” (“other, different”), making it initially a term of opprobrium. Only in the first decades of the 20th century did it settle into its present niche, cushioned with overtones of romance, pleasure, health and normalcy.

Just because there wasn’t a word, obviously, doesn’t mean the concept didn’t exist. And yet, Ms. Blank points out, for much of history it never really needed a definition.

Back in the day, heterosexuality was simply the primal process that perpetuated the species with sex and sorted its possessions with marriage. It did not necessarily require much else, certainly not romance or love. “Specific sexual behaviors, to be sure, were named, categorized and judged,” Ms. Blank notes. But individuals were not: “Sexual misbehavior was not a marker of some sort of constitutional difference but merely evidence of temptation unsuccessfully resisted.”

Then came Linnaeus, with his compulsion to name and categorize living things. Then the growth of the metropolis crowded millions of sometimes very badly behaved individuals in close quarters, mandating systems to address issues like prostitution and an acceptable legal age of consent. Birth control and women’s rights slowly dissociated the relationship between the sexes from its biologic and economic imperatives, leaving individuals with unprecedented behavioral freedoms.

Thus, in Ms. Blank’s construct, emerged a comprehensive desire to define both a heterosexual being and a range of acceptable heterosexual behaviors. But even now, we are still working on both.

Scientifically, as Ms. Blank summarizes, tongue in cheek: “We don’t know much about heterosexuality. No one knows whether heterosexuality is the result of nature or nurture, caused by inaccessible subconscious developments, or just what happens when impressionable young people come under the influence of older heterosexuals.” Far more scientific firepower, in other words, has been directed at the brains, genes, hormones and general physiologic processes behind homosexual attraction, leaving heterosexuality like a silhouette, outlined only by what it is not.

Yet the great behavioral descriptionists, Alfred Kinsey and others, have made it clear that sometimes it is exactly what it is not — or, rather, it is what many feel it should not be. From same-sex adults sharing a bed (for warmth? from friendship?) in the 19th century to married men “on the down low” in the 21st, self-defined heterosexuals have routinely behaved in ways that seem to contradict the basic principles.

But who wrote those principles? Who validated them? Ms. Blank points out that the standards of heterosexuality to which so many desperately aspire have largely been the work of our culture’s biggest dreamers, including the authors of 19th-century penny novels and 21st-century chick lit. Who, after all, has given us more clear-cut, universally appealing examples of suitably behaved male and female heterosexuals than Walt Disney?

Meanwhile, the annals of law are now filling with all the subtleties that Disney ignores, for people who fail to fit into a binary sex/gender system still have both children and property. Empires may rise and fall, but those eternals remain.

Ms. Blank offers the provocative solution that soon we will move on from our present fixation on the binary to a more fluid understanding. “If male and female are two of a variety of sexes, and masculine and feminine two of a variety of genders, then heterosexual and homosexual are two of a variety of ways to combine them,” she notes.

That sentiment will upset some readers. It will strike others as merely logical, a controversial but evidence-based scholarly footnote to the chocolates and pink hearts of the season.

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