Identical twins share nearly all of their DNA, but “nearly all” is the key phrase. A 2021 study in Nature Genetics found that identical twins differ by an average of 5.2 genetic mutations that arise very early in development, before or shortly after the embryo splits in two. Beyond genetics, they also share varying degrees of physical traits, metabolic function, gut bacteria, and psychological characteristics, though none of these are a perfect match.
How Much DNA They Actually Share
For decades, scientists treated identical (monozygotic) twins as genetic carbon copies. That assumption has been revised. When a fertilized egg splits to form two embryos, DNA replication errors can occur during cell division, meaning each twin accumulates small, unique mutations from the very start. Whole genome sequencing studies estimate that roughly 1% of detectable genetic variants differ between identical twins. Most of these differences have no obvious effect on health or appearance, but some can land in genes that matter.
About 10% of new mutations in any person occur after fertilization but before the cells that will eventually become eggs or sperm are set aside. This means some post-split mutations can be passed to the next generation, making the children of identical twins slightly more genetically distinct than you might expect. Around 2.1% of new mutations occur specifically after the twinning event but early enough to appear in both body cells and reproductive cells.
Shared Traits Before Birth
Whether identical twins share a placenta depends on when the embryo splits. About 70 to 74% of identical twin pairs are monochorionic, meaning they share a single placenta. The remaining 26 to 30% each develop their own. Only 1 to 2% share both a placenta and an amniotic sac.
Sharing a placenta doesn’t make twins more similar. In fact, it often does the opposite. A single placenta wasn’t designed to support two fetuses, and blood and nutrient distribution between the twins is rarely equal. Unequal placental sharing is a major driver of birth weight differences in identical twins. In 5 to 30% of placenta-sharing pairs, the imbalance becomes severe enough to cause twin-to-twin transfusion syndrome, where one twin receives too much blood flow and the other too little. Researchers have traced these growth differences to the uneven distribution of specific amino acids between the twins, rather than any general placental malfunction. So even before birth, identical twins can diverge in size and development based on their physical position in the womb.
Height, Weight, and Metabolism
Physical measurements in identical twins converge strongly after infancy. At birth, height correlation between identical twins is only about 0.58 on a scale where 1.0 means perfect agreement. By age two, that correlation climbs to 0.89. Weight follows a similar pattern, reaching 0.87 by one year. The initial gap at birth likely reflects the uneven prenatal environment, while the convergence afterward reflects shared genetics kicking in.
Resting metabolic rate, the number of calories your body burns at rest, also shows a strong genetic signal. In a study of young adult male twins, identical pairs had metabolic rate correlations between 0.45 and 0.81, depending on how the rate was measured. Fraternal twins, by comparison, only correlated between 0.21 and 0.44. This suggests that how efficiently your body burns energy is substantially inherited, which helps explain why identical twins tend to maintain similar body compositions throughout life when living in comparable environments.
Fingerprints Are Not One of Them
Despite sharing nearly all their DNA, identical twins have different fingerprints. Fingerprint ridges begin forming around the 13th week of pregnancy, starting as small indentations on the fingertips. Three families of signaling proteins interact to shape the ridges: one stimulates cell growth to create raised bumps, another suppresses growth to form grooves, and a third determines the spacing and width of the ridges. These proteins are expressed in alternating stripes across the developing fingertip.
The broad pattern of a fingerprint, whether it’s a whorl, loop, or arch, depends on the anatomy of the finger and the precise timing of ridge formation. Ridges can start growing from up to three different spots on the fingertip, and the exact moment each one begins determines how the pattern unfolds. Small, random fluctuations in this process create the fine details that make every fingerprint unique: ridges that stop short, split in two, or form tiny islands. As developmental biologist Roel Nusse put it, fingerprints are “a great example of how minor fluctuations can generate endless variations in a pattern.” This is why even twins who look identical to everyone else can be distinguished by their prints.
Chemical Changes That Accumulate Over Time
Even when twins start with virtually the same DNA, their bodies read that DNA differently as they age. Chemical tags (called epigenetic markers) sit on top of DNA and act like dimmer switches, turning genes up or down without changing the underlying code. In identical twins, these markers start out very similar but drift apart with time.
A large twin study tracking these markers from early adulthood to old age found that chemical differences between identical twins grew by 8 to 16% per decade, depending on the gene. For overall DNA marking patterns, older twin pairs showed nearly double the within-pair differences compared to younger pairs. The vast majority of this drift was driven by unique environmental exposures and random biological noise rather than anything inherited. In practical terms, this means two identical twins at age 20 are far more biologically alike than the same pair at age 70, even if they’ve lived in similar conditions.
Gut Bacteria: Shared Living Matters More
You might expect identical twins to host similar communities of gut bacteria, given their shared genetics. The reality is more nuanced. In a study of 99 twin pairs, the average gut bacterial similarity between any two twins was only about 11%. Roughly half the twin pairs were “concordant,” meaning their microbiomes were more similar to each other than to unrelated people, with similarity around 20% or higher.
The biggest factor driving gut similarity wasn’t genetics. It was whether the twins still lived together. Concordant twins sharing a household had the highest bacterial similarity at about 31%, while discordant twins living apart averaged just 12.9%. Identical twins did show stronger correlations than fraternal twins for certain bacterial groups, but the overall picture was clear: shared environment and aging patterns shaped the gut microbiome far more than shared DNA. As twins aged, the number of bacterial species they shared steadily declined while unique species increased, mirroring the epigenetic drift seen in their DNA.
What This Means for Nature Versus Nurture
Identical twins are the closest thing to a natural experiment for separating genetic influence from environmental influence. The pattern across every trait researchers have studied is consistent: genetics provides a strong starting blueprint, but the environment sculpts the final result. Metabolic rates, heights, and body compositions stay closely matched when twins share similar lifestyles. Chemical markers on DNA, gut bacteria, and disease risk diverge when they don’t.
The old framing of identical twins as perfect genetic clones has given way to something more interesting. They begin life with a handful of unique mutations, develop in a womb that treats them unequally, grow fingerprints that could never be confused, and accumulate a lifetime of molecular differences driven by everything from diet to random cellular chance. What they share is profound, but what makes them distinct starts earlier than anyone suspected.