Jennifer Couzin-Frankel examined how much mortality from various causes would have to drop in order to boost life expectancy and concluded that most of the past increases in life expectancy occurred because of improved survival rates for young people. She states that it seems unlikely that life expectancy at birth will ever exceed 85 years. Michio Kaku argues that genetic engineering, nanotechnology and future breakthroughs will accelerate the rate of life expectancy increase indefinitely. Already genetic engineering has allowed the life expectancy of certain primates to be doubled, and for human skin cells in labs to divide and live indefinitely without becoming cancerous.