There are a few more explosions than usual happening these days. For instance, if you think you're seeing more twins around, you're right. The rate of twin births has increased by 35 per cent during the last 15 years, thanks to the use of fertility drugs and having babies later in life.

    Aviva's father, York psychology professor and child psychologist Robert Muller (BA '87) says that for his part, having twins rocked his world in a way nothing else ever has. "It has blown some of my beliefs right out of the water. Like the notion that life could be predicted, mapped out, planned." Becoming a parent with his first child was a life-changing experience he said, but having twins altered him. "I used to be more anxious, I would contemplate how I could figure out my life, now I'm more willing to let life unfold itself to me. I understand at a very deep level the randomness of life."

    These people who have shared a womb ­ and in the case of identicals, an egg ­ seem to exercise a strange power over singletons, those who come into the world solo. Even strangers on the street can't resist comparing and contrasting them, and probing to find out whether they enjoy a special connection that eludes the rest of us. "Like pregnant women, twins seem to be public property," notes Clark Hortsing-Perna, associate director of York International and father of one-and-a-half-year-old twins, and a daughter, age 4. "Everywhere we go, people make comments and ask questions they would never ask about a singleton. It's as if they feel they have the right to be intrusive."

    Mainstream media delight in reporting on studies of identical twins reared apart who share behavioural and emotional similarities. Indeed, these studies fuel the controversial argument that genes play a much larger role than our environment in determining who we become and the choices we make as adults.

    While that debate rages, twins and their parents say many people fail to understand a key lesson that multiple birth children have to teach. They say to look not at their similarities, but at their extraordinary differences for proof that every human is unique, despite any physical appearances to the contrary.

    York psychology professor Juan Pascual-Leone, an expert in cognitive development, is a triplet who has an identical brother and a fraternal twin sister. Pascual-Leone says he grew up in a tight-knit family of four children, and his brother is his best friend. Still, he says, the mirror lies ­the two aren't that much alike. "My brother is more impulsive, more extroverted, while I am sensitive, quieter, more of a thinker."

    Both studied medicine, inspired by their father, who was a pediatrician. After graduation, Pascual-Leone says they took different routes. He became a traveller and an academic, his brother stayed put and is now a well-established doctor in Spain. His triplet sister became an art history teacher, in keeping with the family-shared interest in the arts.

    Speaking as a triplet, Pascual-Leone is not impressed by studies that make much of twin similarities. "This nature versus nurture argument is simply a current fad," he says. "We are emerging from a long period in which genetic differences were understated, and absolutely everything was interpreted as being a relationship to learning. A correction was needed, and the pendulum is now swinging to the other extreme. I think the truth is that genetic determination sets boundary conditions, and experiences influence the manifestations. It is a case of nurture and nature."

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