Identical twins have been shown to have some amazing psychological similarities. but we should be cautious about attributing these similarities to shared genes because

answer; ///any two strangers are likely to share many coincidental similarities;

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.