It's a widely held saying that an individual's sense of responsibility (i.e. the ability to action if someone is hurt) decreases in a group environment.

This has been shown time and time again, as groups may stand and watch while people are injured, hurt, right in front of their eyes.

So, Is there any clinical evidence or is it just an assumption ?

In psychology it's called the bystander effect (http://en.wikipedia.org/wiki/Bystander_effect).

The research to support its existence is fairly old and was done by Latané and Darley:

Latané and Darley sat a series of college students in a cubicle amongst a number of other cubicles in which there were tapes of other students playing (the student thought they were real people). One of the voices cries for help and makes sounds of severe choking. When the student thought they were the only person there, 85% rushed to help. When they thought there was one other person, this dropped to 65%. And when they thought there were four other people, this dropped again to 31%.

From http://changingminds.org/explanations/theories/bystander_effect.htm

There has been some work since then that has refined our understanding about how people in groups react but their basic finding has not been disproved.

This can also be observed on a daily basis.

Public parks are a good example: There are areas where it is forbidden to walk on the grass. When there are few people there, they will keep to the existing paths and not walk on the grass or flowerbeds. But once there are more people there, and some start walking on the grass, others will more likely do so as well.

Ah, thanks, finally something concrete to back up my assumptions.

This can also be observed on a daily basis.

Public parks are a good example: There are areas where it is forbidden to walk on the grass. When there are few people there, they will keep to the existing paths and not walk on the grass or flowerbeds. But once there are more people there, and some start walking on the grass, others will more likely do so as well.

ii would say thats more a desire that is realised because of the whole "safety in numbers thing" as opposed to lack of responsibility