WARREN, MI - MARCH 04:  Republican presidential candidate Donald Trump speaks to guests during a rally at Macomb Community College on March 4, 2016 in Warren, Michigan. Voters in Michigan will go to the polls March 8 for the State's primary.  (Photo by Scott Olson/Getty Images)
Donald Trump's Achilles' heel: 'Short fingers'
02:07 - Source: CNN

Story highlights

Finger size, rather than hand size may give a clue about a man's genitals

Hormone exposure in the womb may determine finger and penis size

CNN  — 

Republican candidate Marco Rubio wandered into a recent debate perhaps more fitting for the playground than a political battleground when he criticized the size of Donald Trump’s hands. The Republican front runner, now famously, struck back in the debate reassuring voters that there was “no problem” with the size of his hands, or anything else for that matter.

The candidates were playing off an age-old assumption that if a man’s hands were small so must be the size of his penis. The source of Rubio’s comment refers back to a 1988 Spy magazine article that called Trump a “short-fingered vulgarian,” perhaps insulting both his tongue and his manhood.

As a medical story, this is low-hanging fruit, so to speak, but given the pervasiveness of the myth (or reality), we decided to investigate what the science says on the matter. If this were a political reality check story our meter would be stuck on “half true.”

Republican candidate Donald Trump reassures voters there was "no problem" with his hands or anything else for that matter

Sifting through a handful of studies we can conclude that while you won’t see the size of a man’s hands listed on a Tinder profile any time soon, a description of a man’s fingers may actually give you a tiny, and we mean tiny, bit of insight.

One finger, and no, not the middle one, holds the key. If a man’s ring finger is longer relative to his index finger, chances are his penis is slightly longer than average. By “slightly” we mean a difference of centimeters, in a game normally scored by inches.

Here’s how scientists figure this out: Methodology varies slightly from study to study, but most researchers took a man’s total measurements assessing his height and weight and also taking a tape measure to his fingers and his stretched penis. One enterprising group of scientists also measured this phenomenon in rats. Imagine trying to make casual cocktail conversation if that is your job.

All of the studies found that the finger ratio did seem to correspond with penis length. A 2011 Korean study of 144 men undergoing urological surgery found this connection. (They found a similar correlation with height too, at least in the flaccid state, for what it’s worth). Another Korean study in 2015 saw this same connection in newborns. An Egyptian study in that same year examining 2,000 healthy men confirmed the same. It did not find a correlation between weight, height or waist circumference, just the finger ratio. A 2007 Iranian study of 1,500 healthy men provided more evidence to the common methodology.

Now, if you done staring at your or others’ fingers and want to know why, here’s where scientists think you really can blame/thank your mother.

As every boy’s body grows (barring a fundamental developmental issue), so does his penis. It will grow slowly until age 4, at which time the penis will grow steadily until he hits puberty, which is a time of rapid growth. But what may ultimately determine the total length depends on how much the boy was exposed to the hormone androgen when he was in his mother’s womb. This is during a crucial period in development called the “masculinization programming window” during a mother’s pregnancy. That’s about at eight to 12 weeks. Different finger bones also have a different sensitivity to exposure to androgen (and estrogen for that matter).

Rats have finger ratios that are similar to humans

To prove this, scientists tested this theory on rats. If you were ever to get close enough to see one, you’ll notice that a rat’s tiny hands look eerily like a human’s (with a funky manicure). Their finger length ratios are also similar to humans. When researchers blocked a mother’s androgen level during the masculinization programming window of her pup’s development, when the rat pup grew to be an adult, his penis size was reduced from the average penis size. Another rat study found those that were exposed to more androgen in this cycle had slightly longer ring fingers.

Follow CNN Health on Facebook and Twitter

  • See the latest news and share your comments with CNN Health on Facebook and Twitter.

    Are you wondering why in the world scientists would study this phenomenon? Scientists want to know more about reproductive development in general. And urologists also say it’s common for young male patients to come to them concerned about their penis size. Knowing the average length of a penis and about penis development in general can help reassure patients. “There’s no book boys get to explain this is how you are supposed to be,” said Emory University Medical School professor of urology Dr. Chad Ritenour. “It is one of those things I often tell people: Men are very attached to their penises in more ways than one.”

    Even in a presidential election.