Why do some individuals get tattooed while others don’t? Is it because of differences in their genes?
Researchers from the University of Southern Denmark in Odense recently addressed these questions. Their findings, reported in February in the journal Behavior Genetics, showed that differences in an individual’s propensity to get tattooed were not due to nature but because of nurture.
‘Nature’ here refers to an intrinsic, genetically-determined predisposition that affects one’s behaviour. ‘Nurture’ denotes extrinsic factors such as one’s education, culture, family, and peers.
A tattoo is an indelible design registered on the skin by injecting inks and dyes into a skin layer called the dermis. There, the pigment particles become engulfed and subsequently kept in place by cells of the body’s immune system, making them permanent.
In 1991, hikers stumbled on the naturally mummified remains of a man since named Ötzi the Iceman in northern Italy. Scientists found that he lived more than 5,000 years ago and had tattoos. Today, many sports and entertainment celebrities also sport tattoos — as do their fans, and indeed anyone who wishes to bear symbols they consider important or significant to themselves on their person.
Since adding any foreign substance to one’s body is a health risk from a medical viewpoint, studying the long-term effects of tattooing on public health has been an important research focus.
Sorting nature from nurture
If two offspring are born from the same pregnancy, they are called twins. The offspring can be genetically identical or non-identical. Identical twins share all of their genes while non-identical twins share on average only 50% of their genes.
After the father’s sperm fertilises the mother’s egg, the unified cell that is formed is called the zygote. The zygote then develops into an embryo that in turn grows into the baby.
Sometimes two sperm can simultaneously fertilise two eggs to produce two zygotes, and the zygotes can go on to form non-identical twins. On the other hand, identical twins are formed when an embryo from a single zygote splits at an early stage to become two embryos, and each then grows to become a baby.
For this reason, identical twins are also known as monozygotic twins and non-identical twins are called dizygotic twins. Monozygotic twins are always of the same biological sex, whereas dizygotic twins — like any other pair of siblings — can be either of the same or of different sexes.
If both members of a twin pair sport a tattoo, they are further said to be concordant. They are also concordant if both don’t. But if one twin has a tattoo and the other doesn’t, they are said to be discordant.
The researchers behind the new study were curious as to whether there was a ‘greater’ concordance at work in monozygotic twins than in dizygotic twins of the same sex. Their search came up empty, i.e. there was no greater concordance, even despite the fact that monozygotic twins share twice as many genes as dizygotic ones.
The finding suggested that genetic differences have little, if anything, to do with an individual wanting to get tattooed.
Additional findings
The Danish Twin Register (DTR) is a database with details of more than 175,000 twins born between 1870 and 2009. The researchers mailed out a questionnaire to twins recorded in the DTR. They received responses from 4,790 individuals, including from both twins of 1,327 twin pairs.
Of these respondents, 1,058 (22%) were monozygotic, 3,501 (73%) were dizygotic, and the zygosity was unknown for 231 (4.8%). The researchers sorted the twins into three cohorts based on their birth years: oldest (1925-1960), middle (1961-1980), and youngest (1981-2004).
Finally, the researchers asked the respondents if their co-twin also had a tattoo, about the tattoos’ colours and sizes, and the age at which they were first tattooed. The responses revealed that 22% had at least one tattoo. In the 1,327 pairs for which both twins responded, the self-reported tattoo status matched the information provided by the co-twin in 98.5% of the cases.
The researchers reported a marked increase from the oldest to the youngest cohorts, from below 6% to above 30%, in terms of the propensity for getting a tattoo. Evidently tattooing has become increasingly popular in recent times.
The researchers also asked the respondents about their lifestyle factors such as education, smoking, amount of physical exercise, and alcohol consumption. They could correlate tattooing strongly only with smoking. But they also acknowledged limitations in their ability to correlate with alcohol consumption and exercise “because the age at first tattoo and age at which the questionnaire was answered may vary up to several decades”.
More concordance for some defects
In 2021, researchers from the Seoul National University College of Medicine in South Korea reported in the journal BJOG the concordance rates for several different birth defects found in 3,386 monozygotic and dizygotic twin pairs born between 2001 and 2019.
Birth defects affecting the nervous system, the circulatory system, cleft lip/palate, and the urinary system were found to be significantly more concordant in monozygotic than in dizygotic twins. The implication was that aberrant biological development in these systems is primarily due to one’s genes.
On the other hand, the concordance rates did not significantly differ for malformations of the eye, ear, face or neck, the genital organs, and the musculoskeletal system, meaning that for these disorders, extrinsic aspects of the uterine environment, such as maternal nutrition, physiology, smoking, and infections, played a more significant role.
The researchers ended their paper writing: “we have shown empirically that tattooing … is a cultural phenomenon with little to no evidence for genetic influences.”
D.P. Kasbekar is a retired scientist.
Published – March 24, 2025 05:30 am IST