A(n) is a factor thought to be significant for human behavior, which can vary (change) from one case to another.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.        

answer; decipher the motives for why people behave the way they do;

answer; have joints that fail from excess by early the accumulated effects of unnoticed infections,all of these things are possible for a person born without the ability to feel pain;

answer; rhythm test;