Infant and child development are important disciplines within the field of psychology and many words are published each year shaping and growing this body of knowledge. It is hard for even the most dedicated parents to keep up.
Infants learn a tremendous number of skills during their first two years. The trip from the purely reflexive "eat-sleep-poop" stage to running around asking "Mommy, why is the sky blue?" requires learning at a rate unmatched at any other stage of life. Not only are they learning new information, but their brains are physically developing at an astonishing rate and that changes the kind of thought and learning they are capable of. One model for this period of infant development, created by Jean Piaget, is detailed below with the addition of language milestones and will teach you how to understand infant cognitive development. Note that this article covers only cognitive development (infant brain development for thinking and language) and not other types of development such as gross and fine motor skills.
- Birth to 1 month - At this stage of baby development, infants respond purely by reflex (e.g. sucking in response to a nipple). There isn't really any cognitive process in these actions. They imitate observed mouth movements, which is the first language milestone, but vocal cords are too undeveloped to form language-like sounds.
- 1-4 months - Infants start to understand patterns, such as the presence of a bottle means that it is feeding time. Infants start to choose to do things because they anticipate the outcome. They learn trust (if their needs are met) or mistrust (if they are not). They begin cooing and making vowel noises, and by the end of this period, they have started to make some consonant sounds (b, k, m, g, p).
- 4-8 months - Until now all of their actions have been centered around their bodies, but now they start to perform actions on external objects without regard to their physical needs, which is a huge addition to baby milestones. They explore by hand and mouth and begin experimenting with objects. They track objects that leave their direct line of vision. They start to join consonants and vowels to form nonsense words ("gaga"). These words start to adopt the rhythm of the language they hear, and they learn to take turns "speaking" with others rather than just talking all the time. By the end of this period, they start to understand some simple words ("milk") and may start to use words to represent objects. These words may be simple but real words ("mama") or may be nonsense words ("buh" referring to a favorite stuffed animal).
- 8-12 months - Infants start to adopt more sophisticated means-end behavior. They mimic actions such as clapping. They will repeat the same action with different objects to compare results, for example discovering that dropping a block on a hard floor makes a sound while dropping a teddy bear doesn't. They learn that objects don't disappear when they are out of sight (object permanence) and will look for objects that have been hidden. They start to use real words and by their first birthday, they can speak up to a dozen words. They understand far more language than they can speak. By the end of this period, they can point to desired objects and nod or shake their heads in response to questions.
- 12-18 months - Infants continue to explore and experiment through these months of baby development stages, but don't understand that some things can hurt them, such as knives and electric sockets, so baby-proofing should already be in place. Their understanding of object permanence is more advanced and they can find objects that have been hidden and then moved to another place. They can remember images of objects without seeing them, and the same applies to other senses. They start to use different tones of voice to give words multiple meanings. For example, the single word "mama" can mean "Where is mama?," "Mama, come here," or "I love you, mama" depending on the child's tone. Words will often be overgeneralized, such as "cat" being used for any furry animal.
- 18-24 months - Infants look for objects in expected places, such as looking for a toy in the toy chest, even if they didn't see the object put there. They learn scripted routines, understanding that a simple phrase like "let's go for a drive" means going to the garage, then getting in the car, then getting buckled into a car seat, then seeing the driver get in, then hearing the car start, and so on. They gain an understanding of past, present, and future. They learn to group objects into categories, thus recognizing an object as a shirt even though it is different from other shirts they have seen. By the end of the period, they have learned to make things up and engage in pretend play. They start to speak in phrases using nouns, verbs, and adjectives (though seldom other parts of speech). They can express feelings such as "me sad," they understand "me" and "mine." They can call themselves by name and by the end of this stage, they will likely use over 100 words.
As with any development chart, the ages listed are mean or median ages. If your child is lagging in some particular area, that doesn't mean there is a need to panic any more than meeting milestones early means your child is a genius. Albert Einstein didn't start speaking until he was three, and didn't speak well until he was much older. My own sister still wasn't speaking long after the charts said she should have, then one day abruptly started using full sentences.
However having said that, delays in speech and language developmental milestones can be particularly troublesome later in life as in many cases people can't ever catch up. Language delays can also be symptomatic of hearing disorders more subtle than typical infant hearing exams can detect.
As with so many things about your child, it is difficult to find the balance between overreacting and ignoring a potentially serious problem. If you have concerns about your child's development, talk to your pediatrician. A couple of online courses in psychology - specifically, in child development - can also help you learn what to watch for as your child grows.