There's a science behind attraction, one that can be more complicated than physics—and significantly more awkward. Albert Einstein knew this better than anyone. In his thirties, Einstein grew so enamored of his cousin Elsa that he left his first wife for her. He and Elsa were twice related, since their mothers were sisters and their fathers were first cousins. The physicist wrote to his familial flame in a 1914 letter, "Dear little Else, I shall cherish you and be thankful for your love. You have proven that you felt attracted to me as a person and not to the great animal in the newspapers."
A few years later, however, he was ready to call off their engagement. Instead, he professed his love to yet another relative: Elsa's 20-year-old daughter, Ilse. "Albert himself is refusing to take any decision; he is prepared to marry either Mama or me," Ilse wrote to a friend. "I know that A. loves me very much, perhaps more than any other man ever will, he also told me so himself yesterday."
Einstein reverted to Plan A the following year and married Ilse's mother.
It might seem like baffling behavior for a man of his intellectual stature, but it makes perfect sense to evolutionary biologists. From kissing cousins to May-December relationships, from partners who look alike to those who seem entirely mismatched, the forces of attraction can appear random, arbitrary, and inexplicable. But they seem mysterious only because they operate on a primal plane, beneath our conscious awareness. Even the most unusual couplings are based, at some level, on an evolutionary calculus that has emerged over millennia.
I Like You, You're Like Me
Like all animals, we're drawn to partners who resemble us. It's part of a phenomenon biologists call assortative mating: Animals with similar traits mate with each other at rates that cannot be explained by chance. It's why Japanese common toads prefer toads of the same size, and why snow geese, which can be either blue or white, tend to pick a partner of the same color.
Many human couples look alike, too—in some cases, strikingly so.
Holly and Adam Anderson, for example, share the same blue eyes and blonde hair, with such similar facial shapes and features that they could pass as siblings if you didn't know they were married. They met in college. For Adam, it was love at first sight: He called his mother that night and told her he'd met the woman he wanted to marry. Holly took a little longer to reach that point, but not by much.
"I was surprised at how comfortable I was with him. Even when I'd technically just met him, I felt as though I'd known him a long time," she says. They were engaged within two months.
Their shared aspirations won Holly over, she says, along with an entrepreneurial spirit they'd both had since childhood. At 5, she made coloring books, selling them to her friends; in high school, she sewed and sold denim purses. Teenaged Adam, meanwhile, ran a thriving lawn-care service. Now the parents of four children, they renovate fixer-uppers and run a business coaching other parents.
Holly thinks their physical similarities probably played a part in how comfortable she immediately felt with Adam, but at least initially, she didn't see it as a point in his favor. She'd been attracted to men from other ethnic backgrounds in the past and had envisioned marrying someone very different from herself.