Final answer:
Observers likely missed the person in the gorilla costume due to inattentional blindness, as they were focused on counting basketball passes. Nearly half of the study participants failed to notice the gorilla because of the intense focus required for their task. Cultural backgrounds can also influence attention and perception.
Step-by-step explanation:
The most likely reason many of the observers do not notice the person in the gorilla costume is that they were focused on counting the passes. This phenomenon is known as inattentional blindness, which is the failure to notice something that is completely visible because the person was actively attending to something else. In the study by Daniel Simons and Christopher Chabris, nearly half of the participants did not notice the gorilla because their attention was directed at counting the ball passes. Culture also influences what we pay attention to, such as in the study by Masuda and Nisbett, where cultural differences affected the recall of objects in visual stimuli.