A child should be able to identify itself with a doll. This does not mean that a doll has to be the same as he with the same hair or skin color. It is not so simple - but also not so difficult!
A smaller child usually identifies much more with touch, smell and color than specific forms. If really small he is not even conscious of his own limbs, and it takes some time before he can control them, so a doll without too many limbs can be a perfect friend.
When a kid grows, he will correspond more with looks. But be aware of the face! A kid has to be able to connect with a friend, also when it is sad or angry. That is why a good doll doesn't have too much smile on his face! It would seem to be laughing at the child when he doesn't feel like it.
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