How Sydney Sweeney & Marilyn Monroe (and Any Distant Cousins) Are Connected
Recent genealogical records publicized that actress Sydney Sweeney shares ancestral roots with 20th-century icon Marilyn Monroe (born Norma Jeane Mortenson). Like tens of millions of people with early colonial North American lineage, their relationship tracks back 8 to 9 generations to early New England settlers, making them approximately 8th cousins.
While an 8th-cousin bond sounds extraordinary in a celebrity headline, it reflects how exponential population expansion works in human pedigrees. Eight generations back, an individual has up to 256 direct ancestors; nine generations back, 512 ancestors. Because of historical endogamy and limited colonial founding populations, millions of living people share common ancestry through figures from the 1600s and 1700s.
Genealogical vs. Genetic Kinship
You have two family trees: your genealogical tree (everyone from whom you descend) and your genetic tree (the small fraction of ancestors who actually contributed DNA you inherited). Because autosomal DNA recombines and dilutes by ~50% per generation, by the 5th cousin level, there is a ~85% chance you share zero centiMorgans of measurable DNA. At the 8th cousin level, zero DNA sharing exceeds 99.99%. Sweeney and Monroe are genealogical cousins, not genetic matches.
Understanding "Degree" and "Removed"
Cousin numbering (1st, 2nd, 3rd) is determined by the minimum generational distance to the common ancestor minus one: k = min(g_1, g_2) - 1. The term "removed" indicates an unequal generational step: |g_1 - g_2|. For example, your mother's first cousin is your 1st cousin once removed, because you are three generations from the grandparents/great-grandparents while they are only two.
Civil vs. Canon Legal Degrees
In Roman / Civil Law systems, degrees of consanguinity count every step up to the common ancestor plus every step back down (g_1 + g_2). 8th cousins are related in the 18th degree of civil consanguinity. Under Canon Law (used historically by Church jurisdictions), degrees count only the maximum distance to the common ancestor (max(g_1, g_2) = 9th degree).
Frequently Asked Questions
Does Sydney Sweeney share DNA with Marilyn Monroe?
Biologically, it is virtually impossible (probability greater than 99.99%) that Sydney Sweeney inherited any identical-by-descent (IBD) autosomal DNA segments from the common ancestors she shares with Marilyn Monroe. Recombination randomizes chromosome segments, and after 8 to 9 generations, genetic contributions from any single distant ancestor are diluted below 0.0003%, well below the detection limits of DNA sequencing. They are related on paper (genealogically), not genetically.
What is Wright's coefficient of relationship (r)?
Developed by geneticist Sewall Wright in 1922, the coefficient of relationship (r) measures the proportion of alleles shared by two individuals that are identical by descent from common ancestors. For full first cousins, r = 1/8 (12.5%); for second cousins, r = 1/32 (3.125%); for 8th cousins sharing a couple, r = 2 * (1/2)^(18-1) = (1/2)^16 ≈ 0.0015%.
How many 8th cousins does the average person have?
Demographic and genetic models estimate that an average person of European, African, or Asian descent has between 100,000 and 1,000,000 living 8th cousins. When family trees span back to colonial or medieval populations, pedigree collapse (where ancestral lines intersect) ensures that almost everyone from that founding population shares multiple distant cousin ties.