Group Preference Statistics

group_preference.c

// (C) 2026 alwynallam@gmail.com, MIT License

// $ gcc -Wall -O3 group_preference.c -o group_preference && ./group_preference

// see https://www.pewresearch.org/social-trends/2017/05/18/1-trends-and-patterns-in-intermarriage/
// see https://news.harvard.edu/gazette/story/2026/06/why-are-white-black-marriage-rates-so-low/

#include <stdio.h>
#include <stdint.h>
#include <assert.h>

double grand_f_0_1() { // https://stackoverflow.com/a/26867455
  uint64_t  bits;
  static FILE * ur = NULL;
  if(ur==NULL) ur=fopen("/dev/random", "rb");
  assert(1 == fread(&bits, sizeof(bits), 1, ur));
  return bits * 5.421010862427522170037264004349e-020;
}

int main() {
  const int marriages = 1000000; // perform matchmaking until
  const double p11 = 0.1;        // probability that an individual in the majority group will find an individual in the majority group marriageable
  const double p12 = 0.1;        // probability that an individual in the majority group will find an individual in the minority group marriageable
  const double p21 = 0.1;        // probability that an individual in the minority group will find an individual in the majority group marriageable
  const double p22 = 0.1;        // probability that an individual in the minority group will find an individual in the minority group marriageable
  const double p1 = 0.90;        // proportion of the population in the majority group
  int marriage = 0;              // count of marriages
  int marriage1 = 0;             // count of marriages with at least one individual from group 1
  int marriage2 = 0;             // count of marriages with at least one individual from group 2
  int intermarriage  = 0;        // count of marriages that have partners from different groups

  // assumption: marriage occurs when two candidates paired at random find each other marriageable
  while(marriage < marriages) {
    int candidate1 = (p1 < grand_f_0_1()) + 1;
    int candidate2 = (p1 < grand_f_0_1()) + 1;
    int match;
         if(candidate1 == 1 && candidate2 == 1) match = grand_f_0_1() < p11 && grand_f_0_1() < p11;
    else if(candidate1 == 2 && candidate2 == 2) match = grand_f_0_1() < p22 && grand_f_0_1() < p22;
    else                                        match = grand_f_0_1() < p12 && grand_f_0_1() < p21;
    if(match) {
      marriage++;
			if(candidate1 == 1 || candidate2 == 1) marriage1++;
			if(candidate1 == 2 || candidate2 == 2) marriage2++;
      if(candidate1 != candidate2) intermarriage++;
    }
  }
  printf("Of %d marriages, %d are intermarriages (%.1lf%%)\n", marriage, intermarriage, (double)intermarriage / (double)marriage * 100.);
  printf("Of marriages with at least one partner from group 1, %.1lf%% will be intermarriages.\n", (double)intermarriage / (double)marriage1 * 100.);
  printf("Of marriages with at least one partner from group 2, %.1lf%% will be intermarriages.\n", (double)intermarriage / (double)marriage2 * 100.);

  return 0;
}

/*
Findings:
  1. The model is not sensitive to the overall pickiness of the individuals, it just takes longer to run
  2. If the groups are equal size (p1=0.50), and the preference matrix is uniform, itermarriage is 50%
  If the majority is 90% of the population (p1=0.90)
    3. and the preference matrix is uniform (pij=0.1), intermarriage is 18.0%
    4. if majority individuals have a strong preference for majority partners (p11=0.2), intermarriage is 5.3%
    5. if minority individuals have a strong preference for minority partners (p22=0.2), intermarriage is 17.5%
    6. if all individuals have a strong preference for partners from the same group (p11=p22=0.2), intermarriage is 5.2%
    7. if majority individuals have a strong aversion to minority partners (p12=0.05), intermarriage is 9.9%
    8. if minority individuals have a strong aversion to majority partners (p21=0.05), intermarriage is 9.9%
    9. if all individuals have a strong aversion to partners from the other group (p12=p21=0.05), intermarriage is 5.2%
  Note: Findings 6 and 9 are consistent with 1.
	10. With the conditions of Finding 2, of marriages with at least one partner from a given group, 66.7% will be intermarriages.
	11. With the conditions of Finding 3, of marriages with at least one partner from the majority, 18.2% will be intermarriages. Of
	    marriages with at least one partner from the minority, 94.7% will be intermarriages.
	12. With the conditions of Findings 4-9, of marriages with at least one partner from the minority, at least 80% will be intermarriages.
*/