I never understood why Asimov and Heinlein in the twilight of their years had a desire to write fan-fic (fiction composed with the intention of mixing characters and situations of wholly different worlds into one compound mess, huh, reality).

At least Heinlein and Asimov did not get together like Niven and Pournelle and magnify each other’s weaknesses.

From Incest and Foundation:

To Seldon’s surprise, his father was not angry upon catching Hari schtupping his own mother doggy-fashion. True, his father had been raised about the ganja patches of Helicon and taught the wisdom of martial arts, and so he knew of behaviors to which Hari had never been exposed.

In hindsight, Seldon would realize that he had never known anything of his father’s authentic background. Centuries later, with a burst of his astounding intuition, Terminus Councilman Golan Trevize would be the first to realize that fact about the Founder.

Seldon’s father surprised him: “Didn’t you know, Hari? Maureen is my mother too, so you’re laying the pipe to your grandma, who is also your mom.”

“I don’t grok it,” said Seldon, using references from the Water Sharer Orange Book legendarily attributed to Michael Valentine Smith, whose scribe was Jubal Henshaw. (Another clue to Earth’s identity was that Mars was in orbit next to Earth, and Martians matured from a puffball with three eyes on extenable stalks to a humanoid form. Those Martians had given Smith the philosophic insights by which the Galactic Empire drew its Institutional Religion and so the Martian environment would be on a planet adjacent to the true Earth.)

Seldon looked at Maureen whose flame bright hair was in colorful contrast to her creamy skin, all of which was evenly tanned under the G type star cataloged at Trantor as Helicon Primary. “That is the secret I was going to tell you when you had your eighteenth Galactic Standard birthday,”” Maureen explained.

Still disconcerted by his breach of Heliconic taboos, Seldon was doubly afflicted by his newly discovered violation of age guidelines. “Since dad is in his mid forties, then you must be in your sixties.”

Maureen and her husband laughed at their son’s mistake. “You seriously underestimate our ages,” said Lazarus Seldon to Hari. “You know that we’re Howards but what you did not know was that we are of the initial generations of Howards.”

“The first generations of Howards is traditionally more than 10 thousand years before the Galactic Era! Back in the time of Stephen Byerly and Susan Calvin.” Hari was astounded.

“Even earlier than that,” Maureen corrected Hari. “I was born on Old Home Earth, in the last decade of the Nineteenth Century of the Common Era. almost fifty years PreAtomic. I gave birth to your dad in the second decade of the Twentieth Century.”

“I was born Woodrow Wilson Smith,” confirmed Lazarus, “named after a political leader who is now remembered as the god of schoolteachers in a faith that peaked in influence in the Third Century of the Galactic Era.”

Maureen added: “Your dad did a lot of things in his extraordinarily long life. He was the second mate in the first manned spaceflight to Earth’s moon and he called himself Powell when he made the first hyperspatial flight at the end of the third decade of the Twenty First Century of the Common Era.”

In his heart, by his nurture on Tobacco Planet and his exposure to the austure culture of the Frepickers, Hari Seldon had picked up patterns of thought at odds with his parents’ revelations.

Besides having committed what was recorded on his conscience as a sin, Hari finally knew that his parents far exceeded human norms. “But what about me?” Hari questioned.

“We don’t know,” admitted Lazarus with a modest smile. “Over our trysts over the centuries, Maureen and I have had issue, of which you are our 56th child, our twenty-third son. Only a quarter of our offspring are long lived.”

“But we gave you an advantage for sure,” Maureen said. “In the womb, we seeded your brain with the genetic matrix that produced the mind of Andrew Libby, the smartest man we have ever known.”

Lazarus nodded. “You ought to be capable of mathematics that were beyond the capacity of other humans. You have great things in store for you, Hari.”

Hari thought back to his daycare provider, a man named Daniel who hair was brushed back straight. Only after he joined the company of other kids had Hari Seldon realized that calculus was not taught at most daycares. “But by genes and training, you made me unique. Am I a free human or a mutant produced to order?”