Sensorimotor: (Birth to age 2)
- Differentiates self from others.
- Recognizes self as a person and begin to act intentionally.
- Achieves object permanence—realizes that people and things exist even when no longer physically present.
- Points to self in the mirror when asked “Where is Alex?”
- Pushes a button to start a toy.
- Shakes a rattle to make a noise.
- No longer cries when caregiver walks out of the room.
- Moves blanket to find pacifier hidden underneath.
Preoperational: (age 2 to age 7)
- Learns to represent events and objects in various ways.
- Thinking is still egocentric: difficulty taking the viewpoint of others.
- Classifies objects by a single feature.
- Holds block up to ear and says “Hello?”
- Engages in pretend play with baby dolls and bottles.
- Upon seeing that a peer is upset, brings peer a blanket.
- Groups all red blocks together, regardless of size.
Concrete Operational: (age 7 to age 11)
- Can think logically about objects and events.
- Achieves conservation of numbers (age 6), mass (age 7), and weight (age 9).
- Classifies objects according to several features and can order them in a series.
- Understands that a feather cannot break a window because it is too soft.
- Recognizes that five pennies stacked and five pennies spread across a table are the same amount of pennies.
- Organizes all farm animals on one paper and all safari animals on another paper, then pairs the big and small like animals together.
Formal Operational: (age 11 through adulthood)
- Can think logically about abstract propositions and test hypotheses systematically.
- Is concerned with the hypothetical, the future, ideological problems.
- Able to solve abstract algebraic equations.
- Compares self to pictures in a magazine.