Social Intelligence: Why It Is Crucial for Friendships

What Is Social Intelligence?

Social intelligence encompasses the cognitive abilities needed to build and maintain friendships. Brent et al. (2014) identify four core components: individual recognition, social decision-making, understanding third-party relationships, and Theory of Mind.

These competencies are not uniquely human. Sheep recognize former group members after two years of separation. Dolphins remember signature whistles of other individuals for over 20 years. Rhesus macaques forgo juice rewards just to see pictures of other monkeys.

The Social Brain Hypothesis

The hypothesis holds that group living created selection pressure for larger, more complex brains (Dunbar, 1998). Neocortex size correlates across species with social group size — a finding that has been consistently replicated.

For humans, the calculation yields a group size of about 150 — Dunbar’s Number. This does not mean we can only know 150 people, but that we can maintain at most 150 stable social relationships at once. Of those, only five are close friendships.

Mentalizing and Friendship

Theory of Mind (ToM) — the ability to understand others’ intentions and mental states — is the most cognitively demanding aspect of social intelligence. In human children, this ability emerges around age 4 and continues to improve into adulthood.

Crucially, negative childhood experiences with peers impair later ToM performance. Social isolation in monkeys leads to abnormal behavior. Conversely, autistic children with older siblings show better ToM performance. Social intelligence is malleable — and social interaction trains it.

Why the Brain Sets Limits

Every relationship requires cognitive resources: Who is this person? What do they think? What do they feel? How do they relate to others in my network? Each additional person raises the cognitive load exponentially.

The brain regions OFC (orbitofrontal cortex), ACC (anterior cingulate cortex), and the amygdala are involved in processing social information. One study showed that the size of a person’s social network correlates with the size of certain brain areas — and those areas can change through social experience (Sallet et al., 2011).

The takeaway: social intelligence has a biological ceiling, but it is not fixed. Actively maintaining friendships trains the neural circuits required for them.

Can You Train Social Intelligence?

Yes — through social interaction itself. The evidence points to a self-reinforcing cycle: maintaining more friendships trains the cognitive abilities needed for them. Isolation erodes those abilities.

Prosocial tendencies are already present at age 3, but they need to be strengthened through experience. This argues for bringing children into social contexts early — and as adults, maintaining regular contact with friends rather than sporadic check-ins.

Frequently Asked Questions

What is social intelligence?

Social intelligence comprises four cognitive competencies: individual recognition (perceiving people as unique), social decision-making (choosing and maintaining friends), understanding third-party relationships (reading networks), and Theory of Mind (grasping others’ intentions and feelings).

How is brain size linked to the number of friends?

The Social Brain Hypothesis (Dunbar, 1998) shows that neocortex size correlates across species with social group size. Larger brains enable more complex social relationships — in humans, this caps out at roughly 150 stable contacts.

Can you train social intelligence?

Yes. Socio-cognitive skills are present early on but shaped by experience. Autistic children with older siblings show better Theory-of-Mind performance. Conversely, negative childhood experiences impair later ability. Social interaction trains social intelligence — a self-reinforcing cycle.

Why can’t we have unlimited friends?

Because mentalizing — understanding others’ intentions, feelings, and perspectives — is cognitively demanding. Each additional person in the network requires more processing capacity. Dunbar’s Number (~150) marks the limit of what our brain can handle.