One to six months
Children under six months completely identify with their primary caregivers. They don't really think about themselves, only what they immediately need: food, love, and attention. In the first three months, your baby can't even think about tackling the process of forming her own identity. She's too busy trying to gain control over her basic movements and reflexes. You may start to notice the first signs of budding independence at about four months. That's when your baby will discover that she can cry to get your attention. That's one of the first steps in learning that she has an independent will and that how she behaves can have an impact on others, namely you.
Seven to 12 months
At around seven months your baby will realise she is independent of you; this is a huge cognitive leap worthy of celebration. Unfortunately, this new understanding of separateness makes your baby anxious. She's become so attached to you that when you leave her alone, even for a minute, she will burst into tears. She doesn't have the information yet that you will always come back. And sneaking out when her back is turned - when you leave her at nursery, for example - won't help. In fact, it may just make her more afraid that you aren't coming back. Hard as it can be, say goodbye and go while she's watching.
A now famous British study shows exactly how clueless babies are about their own existence. Researchers placed several infants under the age of one in front of a mirror to see whether they understood that the reflection was an image of themselves. They didn't. The children patted their mirror image, behaving as if they were seeing another baby. And when researchers dabbed red rouge on each baby's nose and plopped them back in front of the mirror, they always tried to touch their reflection's nose, not their own.
12 to 24 Months
Your baby's making more progress now differentiating herself from you and from the world around her. In the same British study mentioned above, researchers put rouge on the noses of children about 21 months old. When they looked in the mirror they touched their own nose, showing that they understood that the image in the mirror was a reflection of them.
Two-year-olds may still get upset when you leave them at nursery or with a babysitter, but they recover much more quickly now because they're more secure. Experience, and their budding memory skills, have taught them that you will come back after being gone for a while. Your toddler's trust in you is growing now, because you have continually shown her that you love and care for her. It's that feeling of trust that gives her the confidence to venture out on her own. What signs of independence will you notice now? Your child may insist on wearing her purple pajamas for the fifth night in a row, eating only certain foods, and climbing into her car seat by herself.
25 to 36 months
Between the ages of two and three, a toddler will continue to struggle for independence. She will wander farther away from you as she goes exploring, and she'll continue to test her limits (colouring on the walls, for example, even if you tell her not to). In fact, "I can do it myself" is probably one of the most common refrains parents hear from older toddlers.