A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do identical twins have different fingerprints?

August 21, 1998

Dear Cecil:

An issue came up the other evening over dinner. Do identical twins have different fingerprints? If it's a genetic trait, one would think the twins would have the same fingerprints at birth; yet we are told everyone has unique prints. Please help me so I can get some sleep.

Dear Kraig:

If this is the kind of thing that keeps you up at night, babe, don't ever have teenagers. The sound-bite answer to your question is yes--identical twins have fingerprints that can be readily distinguished on close examination. However, the prints do have striking similarities. In fact, before the arrival of modern genetic testing, similarity of fingerprints was often used to determine whether twins were identical or fraternal. (Identical twins, you'll recall, are genetic duplicates who develop from a single egg. Fraternal twins develop from separate eggs and are no more closely related than ordinary siblings, except that they spend nine months sharing an extremely small bedroom.)

Twin fingerprints are much beloved by scientists, who see them as a classic arena for the old nature-versus-nurture debate: What made you what you are today, your genes or your environment? Twin fingerprints clearly show that it's a little of both. If you compare palm prints and fingerprints of the Dionne quintuplets (born in 1934, they were the first quints of which all five survived), you find that the broad-brush pattern of lines, whorls, loops, etc., as well as what researchers call "ridge count," were quite similar for the whole crew. Nonetheless each kid had unique prints due to differences in detail. "There is as yet no evidence that the arrangement of the minutiae (ending ridges, bifurcating ridges, etc.) is in any way genetically influenced," writes fingerprint expert James Cowger. Presumably these minor but crucial differences arise from random local events during fetal development, the same kind of thing that makes each snowflake unique.

"But Cecil," you say, "you yourself revealed that duplicate snowflakes have been found!" So I did, but don't think that's gonna help you beat a murder rap. One genius has computed that the chances of duplicating even a portion of a fingerprint are 1 in 100 quintillion (one followed by 20 zeros). Multiply that by the totality of each finger times ten fingers and you can see why O.J. thought he'd have better luck hiring a rhyming lawyer. (OK, O.J.'s problem was DNA, not fingerprints, but you know what I'm saying.)

Now think what this tells us about the nature of life. (You thought this was just about fingerprints? And Zen and the Art of Motorcycle Maintenance was just about motorcycles!) Fingerprints suggest we are not simply the prisoners of our genes. On the contrary, much of our physical makeup seems to be improvised--improvised, moreover, not by some master jazz musician, but by a collection of stupid molecules. How can this be? For that matter, how does this whole business of "gene expression" work? I mean, you start off with a little blob of protoplasm, and the cells divide, and somehow one bunch of cells knows it's going to wind up being the liver, another the eyeball, and another the right pinkie . . . but how? How does this fabulously complicated creature arise from the genetic information in the chromosomes? I gotta confess to you, friends. It stumps even me.

Let's close with one question maybe we can answer. Assuming the Creator didn't make fingerprints solely as an aid to law enforcement, what are they for, anyway? James Cowger thinks he knows. He calls fingerprints "friction ridges" and suggests they make it easier to get a grip. And these days, God knows, couldn't we all use the help?

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A Straight Dope Classic by Cecil Adams
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