story identification - A perfect backscratch as evidence of telepathy

A man and his son swim at the beach. Afterward, the man lies down to dry in the sun, and the boy obligingly scratches his back. The salt is making the man's back itch as the seawater dries, and the man suddenly realizes that the boy has to be telepathic, because he's scratching all the right spots just as they start to itch, and no one can do that without reading another person's mind.

This is just a vignette from an English-language novel that I read in the 1960s or 70s. I'm sure that I could remember more about the novel if I could place it in context. What's the book?

Answer

This comes near the end of "Beyond This Horizon" by Robert Heinlein. I can't find a link to post.

On a similar occasion but much later an event occurred which was quite as disturbing, but a little more productive. Felix and the boy had been splashing in the surf, until they were quite tired. At least Felix was, which made a majority with only one dissent. They lay down on the sand and let the sun dry them. Presently the salt drying on the skin made them itch, as it has a habit of doing.

Felix scratched Theobald between the shoulder blades- that awkward spot-and reflected to himself how catlike the child was in many ways, even to the sybaritic way in which he accepted this small sensuous pleasure. Just now it suited him to be petted; a moment later he might be as haughty and distant as a Persian tom. Or he might decide to cuddle.

Then Felix lay on his stomach, Theobald straddled his back and returned the favor. Felix was beginning to feel rather catlike himself-it felt so good!-when he began to be aware of a curious and almost inexplicable phenomenon.

When one human monkey does another the great service of scratching him, delightful as it is, it never quite hits the spot. With infuriating obtuseness, despite the most careful coaching, the scratcher will scratch just above, just below, all around the right spot, but never, never, never quite on it, until, in sheer frustration, the scratchee will nearly dislocate his shoulder going after it for himself.

Felix was giving Theobald no instructions; in fact, he was nearly falling asleep under the warm relaxing ecstasy of his son's ministrations, when he suddenly snapped to attention.

Theobald was scratching where Felix itched.

The exact spot. An area of sensation had only to show up for him to pounce on it and scratch it out of existence.