Research by my University of Chicago colleague Steve Levitt and Julie Cullen of the University of California, San Diego, showed that for every homicide that occurs in a city, total population declines by 70 people.

FORBES: Chicago, Summer Crime, and the Slide Toward Detroit

Among the speakers were US actors Hayek and Matthew McConaughey, award-winning scientist and environmentalist Dr David Suzuki, Canadian astronaut Julie Payette and Bindi Irwin, the young daughter of the late Australian wildlife expert, Steve Irwin.

BBC: 'Space clown' hosts global show