The group is a lever, not just a backdrop
A useful way to think about it: a person's behavior is not fixed and simply revealed in company. Social environments actively modify behavior through several levers — social learning (watching how others act), reinforcement (what gets rewarded), and social influence (pressure to match others) [3]. In small groups, decisions are often shaped by social influence that is either normative (what feels expected) or informational (what others seem to know), and the size of competing factions within the group can tilt which way an individual goes [4].
This means the same person can genuinely behave differently in two groups without being insincere. The group supplies cues about what is appropriate and whom to trust, and those cues feed directly into the choice being made [3][4].
Talking together can make people more generous — and more trusting
A concrete example comes from an experiment with students at 13 schools who were paired up and asked how much of a shared fund to transfer to refugee minors [2]. Some pairs had a free-form chat first; others wrote down their reasoning individually. The pairs who talked expressed more refugee-friendly views than the solo reasoners — the researchers describe this as a political-correctness norm shaping the conversation — and the chat also made each partner more optimistic that the other was refugee-friendly [2].
That optimism mattered. People trusted their partners more the more refugee-friendly they believed them to be, and in this sample that trust was rational, because the more refugee-friendly participants really did turn out to be more trustworthy [2]. Communication also raised willingness to collectively share funds with the refugee minors, so both the in-group and the out-group benefited from the norm of speaking well of the out-group [2]. The scope here is specific: these were school minors in a lab-in-the-field setting, so the result describes that sample rather than all groups everywhere.
Group influence is not one uniform push
It would be a mistake to assume groups always make people more generous or always make them more cautious. The theory work frames behavior change as arising from multiple levers at once — learning, reinforcement, and influence — and notes that individual responses vary [3]. The small-group research likewise treats outcomes as depending on the mix of influence types and the relative sizes of factions within the group, not on "the group" as a single force [4].
So the honest answer to why people act differently in a group is that the group changes the information and expectations a person is working with. Whether that produces more generosity, more conformity, or something else depends on what the group's conversation makes salient and whom the person comes to trust [2][4]. The dog study in the evidence set is a reminder of a different point — that responses to emotional cues vary between individuals — but it examined dogs reacting to human actors, not group settings, so it does not speak directly to human group behavior [1].
Sources used in this answer
How Do Dogs Behave When Presented with Situations of Different Emotional Valences?
Dogs showed different behavior toward human actors expressing positive, neutral, and negative emotions, with more exploration and information-gathering in the positive conditions, though the study did not examine group settings.
How to talk about an out-group: Effects on in-group trust and out-group generosity
In a school-based experiment, pairs who chatted before deciding how much to share with refugee minors expressed more refugee-friendly views, trusted each other more, and were more willing to share collectively than pairs who reasoned individually.
Social Behavioral Theory
The theory identifies social learning, reinforcement, and social influence as primary mechanisms through which social environments modify individual behavior, with variability in how individuals respond.
Shared representations and asymmetric social influence processes in small groups
Small-group decision-making is shaped by normative or informational social influence, and the relative sizes of factions within a group affect how individuals behave.