Another way to look at development is to study different cultures. This allows us to see the vast range of behaviors in humans. Most research that you read is based in the USA and is limited by our own cultural models.
The most usual behaviors studied are in the realm of child rearing and family living arrangements and we find variations that are far outside the normal distributions of those behaviors that we find in the United States.
Child rearing practices
Last year after the tsunami you might have noticed on TV and in the papers news about "baby 81" in Sri Lanka. If you noticed, he had a mark on his forehead, a black "mottu," put there by a nurse to ward off evil. There are many cultures that put a mark somewhere on an infant or child or put an amulet on an arm or leg. Some cultures cover the bed of the sleeping infant with black cloth to ward off evil.
[Some pediatricians in the United States would suggest that our infant talcum powder and whatever else we sprinkle on babies fit into the same category.]
Sleeping
In many cultures infants and young children sleep with their parents. In our culture this is usually taboo as we have some cultural prohibitions about where children sleep and with whom. There is recent research on this topic that does suggest that even in the United States infants should sleep near the parents and even in the same bed.
Weaning and toilet training
In the United States we tend to use a calendar or an aging system to mark when children should do things. If you asked most American parents when their children were weaning or toilet trained they will give you the child's age. Parents sometimes feel that the earlier the child is toilet trained the more "brownie points" they get. This is one of my personal biases. I have see too many cases of child abuse resulting from a child wetting a diaper when the parents thought the child should have stopped doing so.
In other cultures toilet training is just not an issue. This is especially true in warmer climates with houses with dirt floors. There is no stigma attached to when and where you go but like with puppy training the goal is to go outside the house.
Regarding weaning - in many cultures an infant is breast-fed for years. More recently there has been a move in the United States to encourage longer periods of breast-feeding.
The above issues about sleeping and breast-feeding are just a few examples of where cross-cultural findings have affected what is now suggested in the United States. But it takes decades for some ideas to take hold here when they’ve been practiced forever elsewhere.
Another cross-cultural factor is what defines a "family." In the USA we mainly have what is called a nuclear family, a set of parents and their children, all living in one house. This is a combination found mostly in Western cultures. Other cultures have a wider variety of living situations. Living with or next to relatives is quite common; calling all people in your immediate neighborhood or village “family” is found in various parts of the world. In some cultures, parents with many children may share their children with relatives who have none. None of this is “right” or “wrong” - each culture determines it’s own rules and values.
Language
All infants start life being able to make all human sounds. Even that eu French sound and the African click. Where did they go? Well - did your parents keep encouraging you to make those sounds? Do you encourage them in your children?
Toys
In some cultures children play with the adult tools such as machetes; tools that many American parent would cringe at.
Physical activity
Barring illness, children have the capacity to be very active. Yet here we label active children as having ADHD and medicate them [opps yes I have strong biases!] Look at the current crop of Olympians - Americans are no longer in the elite categories in many sports...One could ask if we are deteriorating physically or is it that we no longer allow our children to develop in that area?
Intellect
Cultures differ in their emphasis on school education. Some do everything possible to ensure that the children are well educated - from early on. Some don't - and some say they do and yet they don't.