The bystander effect is a social psychology phenomenon that causes crowds not to offer help to a single person in trouble. The greater the number of bystanders the less likely it is that any one of them will come to help the victim.
Social psychologists Bibb Latané and John Darley popularized this concept following the infamous 1964 Kitty Genovese murder in New York City. Ms. Kitty was raped and stabbed in an incident that lasted for about 30 minutes. Although she screamed and begged for help none of the almost 40 people who have seen or heard the crime stepped in to assist or call the police.
In 2011, another horrible example happened in the city of Foshan, China. Wang Yue, a 2-year-old girl, was hit by a small van. Since she was not moved by any of the 18 witnesses she later got run over by a large truck. A total of 18 people ignored her, some going so far as to walk around her blood without helping the child. The toddler was left for seven minutes before a recycling vehicle picked her up and called for help. She died eight days later.
Although there are several factors contributing to the bystander effect, diffusion of responsibility is considered the most prominent one. If witnesses to an incident are in a large group they assume - or wish - that some of the others will take action. Their individual responsibility becomes group responsibility, and so they just pass by or observe thinking, "If everyone else is passing by or observing, it's okay for me to do the same".
To make things worse, not-so-few people actually enjoy watching (recently even filming) others getting hurt. Being a part of a group diminishes one’s sense of personal responsibility due to a feeling of anonymity. In this state, people are more likely to do things they would never do individually.
It is noted that when someone surrounded by a crowd needs help it can take a long time for someone to interfere, but once the first individual steps in many more will usually follow because they will be "awakened" by the action of the first individual. So be that first one and help!
I would like to conclude this grim post by sharing a short light-hearted experiment made by SoulPancake. Enjoy! : )
Have a wonderful day, week, and life!
Big thanks to @transcript-junky who provided a free post proofread.