The Third Twin by Ken Follett

catchphrase that summed up the argument: “Nature or nurture?”
    She nodded, and her long hair moved heavily, like the ocean. Steve wondered how it felt to the touch. “But we’re trying to resolve the question in a strictly scientific way,” she said. “You see, identical twins have the same genes—exactly the same. Fraternal twins don’t, but they are normally brought up in exactly the same environment. We study both kinds, and compare them with twins who are brought up apart, measuring how similar they are.”
    Steve was wondering how this affected him. He was also wondering how old Jeannie was. Seeing her run around the tennis court yesterday, with her hair hidden in a cap, he had assumed she was his age; but now he could tell she was nearer thirty. It did not change his feelings about her, but he had never before been attracted to someone so old.
    She went on: “If environment was more important, twins raised together would be very alike, and twins raised apart would be quite different, regardless of whether they were identical or fraternal. In fact we find the opposite. Identical twins resemble one another, regardless of who raised them. Indeed, identical twins raised apart are more similar than fraternal twins raised together.”
    “Like Benny and Arnold?”
    “Exactly. You saw how alike they are, even though they were brought up in different homes. That’s typical. This department has studied more than a hundred pairs of identical twins raised apart. Of those two hundred people, two were published poets, and they were a twin pair. Two were professionally involved with pets—one was a dog trainer and the other a breeder—and they were a twin pair. We’ve had two musicians—a piano teacher and a session guitarist—also a twin pair. But those are just the more vivid examples. As you’ve seen this morning, we do scientific measurements of personality, IQ, and various physical dimensions, and these often show the same pattern: the identical twins are highly similar, regardless of their upbringing.”
    “Whereas Sue and Elizabeth seem quite different.”
    “Right. Yet they have the same parents, the same home, they go to the same school, they’ve had the same diet all their lives, and so on. I expect Sue was quiet all through lunch, but Elizabeth told you her life story.”
    “As a matter of fact, she explained the word ‘monozygotic’ to me.”
    Dr. Ferrami laughed, showing white teeth and a flash of pink tongue, and Steve felt inordinately pleased that he had amused her.
    “But you still haven’t explained my involvement,” he said.
    She looked awkward again. “It’s a little difficult,” she said. “This has never happened before.”
    Suddenly he realized. It was obvious, but so surprising that he had not guessed until now. “You think I have a twin that I don’t know about?” he said incredulously.
    “I can’t think of any gradual way to tell you,” she said with evident chagrin. “Yes, we do.”
    “Wow.” He felt dazed: it was hard to take in.
    “I’m really sorry.”
    “Nothing to apologize for, I guess.”
    “But there is. Normally people know they’re twins before they come to us. However, I’ve pioneered a new way of recruiting subjects for this study, and you’re the first. Actually, the fact that you don’t know you have a twin is a tremendous vindication of my system. But I didn’t foresee that we might be giving people shocking news.”
    “I always wanted a brother,” Steve said. He was an only child, born when his parents were in their late thirties. “Is it a brother?”
    “Yes. You’re identical.”
    “An identical twin brother,” Steve murmured. “But how could it happen without my knowledge?”
    She looked mortified.
    “Wait a minute, I can work it out,” Steve said. “I could be adopted.”
    She nodded.
    It was an even more shocking thought: Mom and Dad might not be his parents. “Or my twin could have been adopted.”
    “Yes.”
    “Or both,