tangles Twisted masses of threads made of a protein called tau within the neurons of the brain; a normal brain contains some beta-amyloid tau, but in brains of people with Alzheimer disease these tangles proliferate, especially in the hippocampus, a brain structure crucial for memory.

temperament Inborn differences between one person and another in emotions, activity, and self-regulation. It is measured by the person’s typical responses to the environment.

teratogen An agent or condition, including viruses, drugs, and chemicals, that can impair prenatal development and result in birth defects or even death.

terror management theory (TMT) The idea that people adopt cultural values and moral principles in order to cope with their fear of death. This system of beliefs protects individuals from anxiety about their mortality and bolsters their self-esteem, so they react harshly when other people go against any of the moral principles involved.

tertiary circular reactions The third of three types of feedback loops in sensorimotor intelligence, this one involving active exploration and experimentation. Infants explore a range of new activities, varying their responses as a way of learning about the world.

tertiary prevention Actions, such as immediate and effective medical treatment, that are taken after an adverse event (such as illness or injury) and that are aimed at reducing harm or preventing disability.

testosterone A sex hormone, the best known of the androgens (male hormones); secreted in far greater amounts by males than by females.

theory of mind A person’s theory of what other people might be thinking. In order to have a theory of mind, children must realize that other people are not necessarily thinking the same thoughts that they themselves are. That realization seldom occurs before age 4.

theory-theory The idea that children attempt to explain everything they see and hear by constructing theories.

thesis A proposition or statement of belief; the first stage of the process of dialectical thinking.

threshold effect In prenatal development, the point at which a teratogen is relatively harmless in small doses but becomes harmful once exposure reaches a certain level (the threshold).

timeout A disciplinary technique in which a child is separated from other people for a specified time.

transient exuberance The great but temporary increase in the number of dendrites that develop in an infant’s brain during the first two years of life.

Trends in Math and Science Study (TIMSS) An international assessment of the math and science skills of fourth- and eighth-graders. Although the TIMSS is very useful, different countries’ scores are not always comparable because sample selection, test administration, and content validity are hard to keep uniform.

trust versus mistrust Erikson’s first crisis of psychosocial development. Infants learn basic trust if the world is a secure place where their basic needs (for food, comfort, attention, and so on) are met.