Natalie Portman has urged Harvard University graduates to take more "risks". The actress star has admitted if she had realised how difficult and how "woefully unprepared" she was to dance the ballet scenes in 'Black Swan' she might have declined the role, but she strongly believes success can only come from testing yourself. Speaking to graduates at Harvard Univeristy on Wednesday, she said: "The point is, if I had known my own limitations, I never would have taken the risk. And the risk led to one of my greatest personal and professional achievements." She added: "Make use of the fact that you don't doubt yourself too much right now because, as we get older, we get more realistic. Accept your lack of knowledge and use it as your asset."