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So when he decided to leave to join a start-up making trainers out of wool, friends and family were bemused.
“They naturally said I was quite dumb,” he says. And that was among the more polite reactions.
Why would anyone want to go into an industry where giants like Nike and Adidas deploy vast marketing budgets and roll out thousands of designs a year?
But for Mr Zwillinger and his co-founder Tim Brown, the creation of Allbirds was not quite the crazy leap of faith that it seemed back in 2014.
The economics of making and selling trainers (or sneakers to many outside the UK) had been changing.