Calculating the Threshold for Altruism

In the scenario:

• Benefit to B (B): 160 units

• Cost to A (C): 50 units

Hamilton’s rule predicts A should help B if:

r×160>50  ⟹  r>50160=0.3125r×160>50⟹r>16050=0.3125

So, A should help B only if their genetic relatedness is greater than 0.3125.

Coefficient of Relatedness for Family Members

• Brother or Sister: $r=0.5$

• Nephew or Niece: $r=0.25$

• First Cousin: $r=0.125$

• Uncle or Aunt: $r=0.25$

(Source: )

Who Should A Help?

• Brother or Sister: $r=0.5>0.3125$ (Yes, should help)

• Nephew or Niece: $r=0.25<0.3125$ (No)

• First Cousin: $r=0.125<0.3125$ (No)

• Uncle or Aunt: $r=0.25<0.3125$ (No)

Therefore, according to Hamilton’s rule, A should help B ONLY if B is his brother or sister.

Conclusion

Hamilton’s rule demonstrates that altruism is most likely to evolve between close kin. In this scenario, only siblings (brother or sister) have a high enough genetic relatedness to justify the cost of helping.

Correct answer: (1) brother or Sister.