What marks out individuals fromeach other? Is it possible tochange personality overnight?Neil Massey/Millennium Images, UKOur Brains, Our SelvesMasud Husain (Canongate Books)What makes us who we are? Most of us ask this question at least once in our lifetimes. Personally, I wonder about it all the time. As an extrovert, much of my identity is built around being outgoing and friendly: these traits make me, me. But where do they come from? How durable are they? Could I wake up one day as someone who is reserved, quiet and calm? It is an unnerving thought.