# Social Behavior and Altruism

Psychology I · Ethology · https://tryals.app/learn/psychology-i/social-behavior-and-altruism

## The Puzzle of Altruism

**Altruism** is any behavior that increases another's fitness while decreasing the actor's own. Natural selection should favor selfishness, making this a major puzzle.

**Hamilton's Rule** resolves this via **kin selection**. Altruism is favored when $rb > c$, where:

| Symbol | Meaning |
| --- | --- |
| **r** | Genetic relatedness |
| **b** | Fitness benefit to recipient |
| **c** | Fitness cost to actor |

Helping a relative spreads your own genes indirectly through shared ancestry.

> **Common pitfall:** **Altruism** in biology requires no conscious kindness. It is defined purely by fitness consequences, regardless of intent.

## Inclusive Fitness & Reciprocity

**Inclusive fitness** is an individual's direct reproductive success _plus_ indirect success achieved by helping relatives reproduce, weighted by $r$. A sterile worker bee achieves high inclusive fitness by raising related sisters.

Beyond kin, **reciprocal altruism** explains cooperation among _unrelated_ individuals. Help is favored if the cost is low, the benefit is high, and the favor is likely to be returned.

| Type | Key Driver | Example |
| --- | --- | --- |
| **Kin Selection** | Genetic relatedness ($r$) | Social bees raising siblings |
| **Reciprocal Altruism** | Expected future return | Vampire bats sharing blood |

## Practice questions

4 of this lesson's 11 practice questions, with answers. The full set is in the app.

### 1. Haldane joked he would lay down his life *"for two brothers or eight cousins."* Why those particular numbers?

A. Because eight is the largest number of cousins a person typically has
B. Because the numbers are arbitrary and the quip is about family loyalty rather than genetics
C. Because brothers are four times more likely to reciprocate than cousins
D. Because relatedness is 1/2 to a brother and 1/8 to a cousin

**Answer:** D. Because relatedness is 1/2 to a brother and 1/8 to a cousin

**Why:** Two halves and eight eighths both come to one, exactly the genetic stake you would be spending. It is Hamilton's rule as a bar-room line, and the reason altruism toward kin needs no special explanation: from the gene's point of view it is not altruism at all.

Page: https://tryals.app/practice/psychology-i/social-behavior-and-altruism/haldane-joked-he-would-lay-down-his-life-for-two-brothers-or-eight

### 2. Sort each example by the mechanism that best explains the helping.

**Answer:**

- Kin selection: A sterile worker bee raises her highly related sisters, A ground squirrel's alarm call mainly warns its nearby relatives
- Reciprocity: A vampire bat feeds an unrelated roost-mate that will return the favor, Two unrelated primates take turns grooming each other

**Why:** Helping relatives (worker bee, squirrel alarm calls) is kin selection, spreading shared genes; helping unrelated partners who repay (bat feeding, mutual grooming) is reciprocal altruism. Relatedness versus reciprocation is the discriminator.

Page: https://tryals.app/practice/psychology-i/social-behavior-and-altruism/sort-each-example-by-the-mechanism-that-best-explains-the-helping

### 3. Select every statement that is TRUE about the evolution of altruism.

A. Hamilton's Rule is rb > c.
B. Altruism requires the actor to consciously choose to help.
C. Inclusive fitness equals direct reproduction plus indirect success through relatives.
D. Reciprocal altruism can occur between non-relatives.
E. Reciprocal altruism is favored even when there is no chance the favor is returned.

**Answer:** A. Hamilton's Rule is rb > c.; C. Inclusive fitness equals direct reproduction plus indirect success through relatives.; D. Reciprocal altruism can occur between non-relatives.

**Why:** The inclusive-fitness definition, non-relative reciprocity, and Hamilton's Rule are all true. Altruism needs no conscious choice, and reciprocal altruism requires a realistic chance of repayment - so those two are false.

Page: https://tryals.app/practice/psychology-i/social-behavior-and-altruism/select-every-statement-that-is-true-about-the-evolution-of-altruism

### 4. Match each concrete case to the concept it illustrates.

**Answer:**

- A sterile worker bee raising her many sisters → Inclusive fitness / kin selection
- Vampire bats sharing blood with roost-mates who reciprocate → Reciprocal altruism
- Helping favored when relatedness-weighted benefit beats the cost → Hamilton's Rule
- An alarm call that endangers the caller but warns the group → Altruism (by fitness consequences)

**Why:** A sterile helper illustrates inclusive fitness, reciprocating bats illustrate reciprocal altruism, the rb > c condition is Hamilton's Rule, and a costly alarm call is altruism defined by its fitness consequences. Each case fingerprints one concept.

Page: https://tryals.app/practice/psychology-i/social-behavior-and-altruism/match-each-concrete-case-to-the-concept-it-illustrates