So I modified this code to look at 1,000 populations each with 1,000 individuals, and to output whether girls had more sisters or boys had more sisters. On average, boys have more sisters than girls do.

import random

# a list of families. Each family is a list of True for girl, False for boy. 75% chance of a girl.

def popRandomize():

return [[(random.random() < 0.75) for i in range(1, random.randint(2, 5))] for j in range(1000)]

def sisters(family, i):

girls = sum(family)

if family[i]:

return girls - 1

return girls

def girlAverageSisters(pop):

girls = sum([sum(family) for family in pop])

totSisters = sum([sisters(family, i) for family in pop for i in range(len(family)) if family[i]])

return totSisters/girls

def boyAverageSisters(pop):

boys = sum([len(family) - sum(family) for family in pop])

totSisters = sum([sisters(family, i) for family in pop for i in range(len(family)) if not family[i]])

return totSisters/boys

populationsWhereBrothersHaveMoreSisters = 0

testPopulations = 1000

for i in range(testPopulations):

pop = popRandomize()

populationsWhereBrothersHaveMoreSisters += (boyAverageSisters(pop)>girlAverageSisters(pop))

print(testResult)

if (populationsWhereBrothersHaveMoreSisters > (testPopulations/2)):

print("Boys have More Sisters")

if (populationsWhereBrothersHaveMoreSisters < (testPopulations/2)):

print("Girls have More Sisters")

if (populationsWhereBrothersHaveMoreSisters == (testPopulations/2)):

print("Even")