In middle childhood, children depend more on external rewards and punishments and struggle to regulate their own behavior

Compared to formal concepts learned in science and math, natural concepts tend to be _____

a) easier to learn. b) very clear and well-defined. c) fuzzy with unclear boundaries. d) good fits with a rigid classification system.

The problems associated with twin studies and adoption studies

a. are not serious enough to cause concern. b. can be minimized by using both kinds of studies to see if they yield similar results. c. can be minimized by using only one kind of study, so potential flaws are not multiplied. d. are insurmountable.