When your classmates at school work cooperatively on a project, it is an example of a positive influence by

Your heredity
Your culture
A peer group
A psychologist

A peer group

When your classmates at school work cooperatively on a project, it is an example of a positive influence by a peer group.

When your classmates at school work cooperatively on a project, it is an example of a positive influence by a peer group.

To arrive at this conclusion, we can examine the options provided and reason through them:

1. Heredity: Heredity refers to the genetic traits and characteristics inherited from our biological parents. It does not directly influence how classmates work together on a project.
2. Culture: Culture refers to the customs, beliefs, and practices of a particular society or group. While culture can shape our attitudes and behaviors, it does not necessarily determine how classmates work together on a project.
3. A psychologist: A psychologist is a professional who studies human behavior and mental processes. Although a psychologist can provide guidance and support, their direct influence on how classmates work together on a project is minimal compared to other factors.
4. Peer group: A peer group consists of individuals who are similar in age, status, and interests. When classmates work cooperatively on a project, they are likely positively influenced by their peer group. The group dynamics, shared goals, and mutual support within the peer group can promote effective collaboration and contribution towards the project.

Hence, the option "A peer group" is the most appropriate answer because it directly reflects the positive influence that classmates can exert on each other when working together on a project in school.