Surely the ultimate expression of sibling­hood, in writing and psychology alike, must be the Jameses, William and Henry (and Alice). William, you’d agree, is a typical Older: commanding lecturer, seemingly confident leader, great stylist. But do we not sense, in his over-bright vision, his too accepting, almost too sunny view of the psyche, the darker depths that we know were there in his own soul? Even when we are in the late, dank novels, doesn’t Henry give more insight, even testable insight, into why people act as oddly as they do? In Henry’s novel “What Maisie Knew,” isn’t the young Maisie closer to the children you study than the children William presents? Would you buy this ultimate sibling paradox: that William is, at times, the better writer, and Henry the better (in some ways the more scientific) psychologist? And if you do — if science can, as you suggest, tell us of climates but can’t describe a cloud — what good is that research for writers like Kluger, or the rest of us, trying so hard to make sense of his own history? To borrow a phrase from William J. himself: What’s the cash value of psychology if it can’t tell us why we feel the way we do and what made us what we are? Why have William at all? Why not just read Henry?

SOS: I think you can answer that question by looking to a very different kind of developmental science. The best and most interesting developmental psychology, especially recently, is not about how children turn out as adults. It’s about what we adults were like as children. We’re like the natural historians who want to know about birds for their own sake — not because they may eventually become chicken dinners. Most developmental psychologists simply want to describe and explain this wonderfully weird, unexpected and gloriously human phenomenon we call childhood.

The influences that make particular children into particular adults are, as you say, largely either obvious or irremediably obscure. It’s much more interesting to explain childhood itself — a period that is much longer for humans than for any other animals. What Maisie knew, or what Henry knew for her, is that childhood is a fascinating mix of innocence and cruelty, brilliant intelligence and painful ignorance, expanded consciousness and narrow experience. And not just your childhood or mine but childhood in general. That is the sort of large, impersonal fact we can start to understand and explain scientifically.

From that perspective, the memoirist’s question takes on a different character. Childhood makes us what we are in a way that is not so much causal as logical. My childhood, for better or worse, is, by definition, part of me. Just as the “I” writing this paragraph includes me at this moment and me next week and me 20 years from now, it also includes the self-confident 5-year-old girl sharing just about everything with her sensitive 4-year-old brother.

SYB: So what we share with brothers and sisters is, in every sense, our DNA: not just elements of the real, physical double helix, but also the more metaphoric helix, the twisted-together nucleus of references, humiliations, ambitions, smells and sources of light that form what used to be called a soul. Ours is a bookish family, but even if the referents are tire swings and swimming holes and ponds more than the first time I met Daisy Buchanan or felt sorry for Pnin, still it is that panoply of reference that shapes my thoughts and feelings. (Our rivalry, at its not infrequent best, took the form of division: London, Nabokov, science and Billie Holiday for you; Paris, Auden, writing and Sinatra for me. I hadn’t read “Pale Fire” until last summer, when I realized that I thought I had because you had read it so intently when we were young.) We know each other better than anyone else, and at the same time know less than many others about how the day works — have intimate knowledge of the foundations, even of what’s at the bottom of the bird cage, though we may live in complete ignorance of the patterns and rituals of the day.

SOS: Yes, and from that perspective siblings loom doubly large. For if parents are the fixed stars in the child’s universe, the vaguely understood, distant but constant celestial spheres, siblings are the dazzling, sometimes scorching comets whizzing nearby. And while parents can participate in childhood in an anxious, vicarious way, siblings are there fully, as fellow children. Siblings are the guarantors that the private childhood world — so unlike the adult world that scientists are only just beginning to understand it — is a fully shared and objective one.

That is why even the scattered, separated six Gopnik sibs or their Kluger counterparts, gathering occasionally at weddings and births and funerals, have an unshakable common ground. Our most beloved and intimate partners, and certainly our own parents and children, will never quite appreciate it the way our siblings do. It’s not just that we all experienced the glorious Christmas when we watched the sparklers light up the dark back hallway, or, in Kluger’s grimmer case, that he and his brothers shared the day when they hid the baby in a fuse cabinet, away from their threatening father. It’s that we experienced those events with the same vision, both exalted and skewed, the sometimes almost hallucinatory vision of childhood.

SYB: Ah!

Now I understand it all.

SOS: Yes, well, I thought you’d get there eventually.