_____ _____ can be positive or negative, occur at irregular intervals, and create stress

From birth until age 2 years, the density of dendrites in the cortex increases

twofold. threefold. fivefold. tenfold.

According to the article Zipping Up the Genes about twins reared apart, which group of people was used

as a control to be sure that the findings of the study were not only the result of having been separated from family (a) a large class of introductory psychology students (b) the researchers in the lab where the twin studies were conducted (c) the spouses of the twins being studied (d) a sample of people located by random dialing of phone numbers