Krista and Blaire are identical twins that were separated at birth. Both of the girls are scared of thunder when they are young. Krista's adoptive parents teach her to deal with her fear by pretending that the thunder is a big drum in an orchestra. Blaire's adoptive parents, on the other hand, teach her to deal with her fear of thunder by thinking about sunny days ahead.

Both girls have inherited a fear of thunder. The difference in their way of dealing with this fear, however, can best be explained by differences in their _______.