The bystander effect is a phenomenon in which people present around the distress are less likely to provide help.
Step-by-step explanation:
This may happen due to the presence of many people in the crowd thus create diffusion of responsibility. In many cases, the victim suffers much due to no attention or help from the people present around.
Another reason for this effect is many times it is considered wrong to help the victim as no other person is doing so thus to be socially accepted in the group of others.