This is a quote from Herb Kohl which puts in a nutshell what we all know about having preconceived ideas, deficit theorising etc. It comes from the book 'Awakening the Heart of Teaching'.
"Looking at a child, understanding who he, she is or might become, is not a simple neutral act or a matter of finding the right objective test or experimental situation. Central to what you see in someone is what you are looking for. If you want to find a child's weaknesses, failures, personal problems or inadequacies you'll discover them. If you look at a child through the filter of her or his environment or economic status and make judgements through the filters of your own culture, gender and racial biases, you'll find the characteristics you expect. You'll also find yourself well placed to reproduce failure and to develop resistance in some children, a false sense of superiority in others. On the other hand if you look for strengths and filter the world through the prism of hope, you will see and encourage the unexpected flowering of a child's life in the most unlikely places"