The prenatal period is divided into three phases:

1. Zygotic phase (0-2 weeks)
   In this phase the sperm has just joined the egg, combining the genes.
2. Embryonic phase (3-8 weeks)
   In this phase, all major organ systems develop.
3. Foetal phase (9 weeks – birth)
   In this phase, the body grows and the organs are refined.

Teratogens are environmental agents that cause harm during prenatal development. The embryo is most susceptible to teratogens. Nutrition and maternal stress are involved in prenatal development. A child physically develops a lot during the early stages of life, especially during infancy and puberty. The head grows a lot first and then the body follows. This is called cephalocaudal development. The average age of menarche has decreased over the last couple of centuries, mainly because of better nutrition.

Infants show signs of habituation and dishabituation. An infant will look at novel stimuli longer than at familiar ones. If an infant is looking at a familiar object and is presented with a novel object, the viewing time is immediately increased. This is called dishabituation. Infants show an increased interest in objects that they can control.

Infants will first put everything in their mouth as a mean of exploring. From the age of 5 to 6 months, infants will start examining objects. They will use their hands and their eyes to look at objects. Here, the rule of habituation also applies. Infants learn about objects’ properties through examination.

The infants also respond to social cues. This can be seen in the phenomenon known as gaze-following and later in life, they will also see other people as intentional agents, individuals who cause things to happen. This is seen in infants of around 9 months of age when they engage in shared attention with another person. They pay attention to the thing the individual points at. Infants will also engage in social referencing. They will look at their caregiver’s emotional expression for clues about the possible danger of their actions.

If infants are shown events that don’t seem fitting with their ideas on the physical world, then they will look at it longer than events that do, showing that even infants have knowledge of core physical principles, such as that unsupported things should fall down.

Piaget argues that infants don’t have a sense of object permanence, which is shown by his simple-hiding experiment, but it may have something to do with having to act on the object, as the infants do show a sense of object-permanence if they only have to look at certain objects, instead of acting on them.

There are three general theories of children’s mental development, starting with Piaget.

Piaget: Mental development derives from the children’s own actions on the physical environment. Children develop schemes, mental blueprints for actions. A scheme is something that a child can do with an object or a category of objects. The growth of schemes involves two complementary processes:

1. Assimilation
   This is the process by which new experiences are incorporated into existing schemes.
2. Accommodation
   This is the process by which existing schemes expand or change somehow to accommodate the new object or event.

Children behave like little scientists and maximise their own mental growth. Operations are actions that can be reversed by other actions. These actions are the most conducive to the mental development of children. They eventually develop operation schemes.             

In Piaget’s theory, there are four stages of development:

1. Sensorimotor stage
   Thought and actions are the same.
2. Pre-operation stage
   Children are now able to use symbols and there are no relations between observations (e.g: volume and width).
3. Concrete-operational stage
   There are relations between observations. Thinking is egocentric, everything the children know must apply to everyone. Children start to think about reversible consequences of actions.
4. Formal-Operational stage
   Children learn abstract principles that apply to a wide variety of objects, also known as formal-operational schemes. They can think about thinking and think about things that ‘could be’.

The knowledge that an entity can stand for something other than itself is representational insight. Centration is fixing the vision on one property of an object and decentration is viewing everything at the same time (e.g: one tall and one wide glass contain the same amount of water).

Vygotsky states that development first occurs at the social level and then at the individual level. People learn to converse with words before they learn to think with words. The zone of proximal development is the realm of activities that a child can do in collaboration with more competent others, but cannot yet do alone. Scaffolding occurs when experts are sensitive to the abilities of a novice and provide responses that guide the novice to gradually increase the understanding of a problem. In Vygotsky’s view, a child is like an apprentice.

The information-processing perspective explains children’s mental development in terms of operational changes in basic components of their mental machinery. Infants are able to form implicit memories, but not explicit memories, because young children must develop the ability to encode their experiences into words before they a form episodic and thus explicit memories. Deferred imitation refers to reproducing the behaviour of a model some significant time after watching the model. Deferred imitation requires symbolic representation.

Executive functions, working memory, inhibition and switching improve steadily throughout childhood. The working memory improves fast but children find it often very difficult to inhibit their speech or to switch between tasks. This improves over the years. The mental-processing speed also increases over the years and is linked to physical maturation and maturation of the brain.

The theory of mind is attributing emotions, motives, feelings, desires, goals, perceptions and beliefs to people and using these attributes to explain their actions. Children are quick to learn that people may want something different than what they want (e.g: 2-year-olds will give broccoli over crackers to an adult if the adult had previously said that he liked broccoli, even though the 2-year old’s preference was crackers). Children have greater difficulty in understanding that someone’s beliefs may be different from their own belief and understanding that someone can believe something that is not true. Young children think that everyone knows what they know (and thus they are unable to fool people).

Young children do understand pretence (pretend play). The difference between pretence and false beliefs are that in pretence everyone knows that is not in compliance with reality, wherewith false beliefs people actually think that it is in compliance with reality.

People with autism or autism spectrum disorder (ASD) are to some degree oblivious to the minds of others. People would serve the same function to you as inanimate objects or machines.

Human language is symbolic and grammatical. The symbols in a language are called morphemes, defined as the smallest meaningful units of a language. Morphemes are both arbitrary and discrete. They are arbitrary in that no similarity need exists between its physical structure and that of the object or concept for which it stands. A morpheme is discrete in that it cannot be changed in a graded way to express gradations in meaning. Morphemes can be broken down into elementary vowel and consonant sounds called phonemes.

All languages are in a way hierarchical, with the sentence up top, and the phonemes all the way down. The rules of a language are called grammar. Grammar consists of rules of phonology, which specifies how phonemes can be combined to form morphemes, morphology, which specifies how morphemes can be combined to form words and rules of syntax, which specify how words can be arranged to produce phrases and sentences.

Grammar is learned implicitly. We use it every day, but (most of us) can’t state all the rules, so it is implicit rather than explicit. Infants that are 6 months old begin to learn to discriminate between phonemes that are different in their own language (e.g: English), but become worse at discriminating phonemes that are not different in their own language (but could be in a different language). Word comprehension precedes word production in infants.

Mutual exclusivity assumption refers to the tendency of young children to link new words with objects for which they do not already know a name. Syntactic bootstrapping is using tactic knowledge of grammar to help them infer the meaning of new words, including verbs and other parts of speech as well as nouns. Taxonomic assumption happens when children learn a new word and they use the word for things that are perceptually similar.

Humans are biologically prepared for language, because we are born with the proper anatomical structures in the throat, brain areas specialized for language, a preference for listening to speech and an ability to distinguish among the basic speech sounds of any language and mechanisms that cause us to exercise oud vocal capacities through a period of cooing and babbling.

Specific grammatical rules vary from language to language, they are all based on certain fundamental principles referred to as universal grammar, that are innate properties of the human mind. The term language-acquisition device refers to the entire set of innate mental mechanisms that enable a child to acquire language quickly and efficiently.

A pidgin language is a language without grammar. A full language with grammatical rules is called a creole language.

Evidence for the LAD comes from patients with brain damage, the development of pidgin languages into creole languages and the fact that deaf people are able to come up with grammar themselves. The most critical period for learning grammar-based languages are in the first ten years of life.

Normal language development requires not just the LAD but also the LASS, the language acquisition support system, provided by the social world in which the baby is born. When someone is speaking to an infant, it is often different than when speaking to an adult and this is known as infant-directed speech. This helps the infant learn the language more easily.