Harry Harlow showed the importance of attachment in monkeys. They became abusive parents when they grew up. Intellect grows through assimilation and accommodation . Assimilation means using existing mental frameworks and patterns to understand a new information. Accommodation means that existing mental frameworks and patterns are modified to understand new information.
Intellectual development is mostly nonverbal.
Do not yet understand cause-effect relationships. Also, egocentric , unable to take the perspective of another person. Child masters conservation . This means that volume, mass, or number don't change even when the shape or appearance changes. Child is able to think abstractly and to consider hypothetical situations.