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Is Penis Size Genetic?

This is who you have to thank for your endowment.

Penis size is, in fact, hereditary. But what determines penis size? Is penis size genetic? And how does a person end up with a micropenis? As it turns out, these questions — as well as questions of where penis genes come from and how much the environment plays a role in who is well-endowed and who isn’t — aren’t exactly straightforward. Studies suggest that penis size is a joint genetic effort between both parents. Although most of the genes responsible for penis size live along the X chromosome, “there are some genes in the Y chromosome that have links to penile lengths and size,” urologist Jamin Brahmbhatt, MD, tells Fatherly. “So you can’t entirely blame your mom for your small penis.”

What Determines Penis Size?

The fetus doesn’t have a penis for its first seven weeks in the womb. At eight weeks, genitalia starts to develop and differentiate. If you have a strong memory of seventh grade health class, you’ll remember that those given a Y chromosome start to grow a penis.

But when it comes to penis size, scientists aren’t sure which parent has more of an influence. Considering genetic brothers can have vastly disparate penis sizes, some experts suspect that there’s more influence from a mother’s two X chromosomes. If size was entirely from the Y chromosome, men with the same father would all have essentially the same size penis. But because size is more likely influenced by the X chromosome, it’s possible for one son to inherit penis size genes from one X chromosome, and one from the other. One brother could find himself well-endowed while the other could be on the smaller side.

And although penis size is largely hereditary, there’s a healthy mix of nature and nurture (aka: one’s environment) at play. A mother’s exposure to chemicals such as phthalates, as well as drugs and alcohol, can impact the size of the child’s penis. But when a baby is born with a small penis due to environmental factors, the penis size is typically the least pressing medical issue at hand.

In Brahmbhatt’s experience, the most common health problem related to penis size occurs when infants do not produce enough testosterone on their own. This can lead to a micropenis, which is defined as a penis that’s less than three inches in length. It can be hard to distinguish between a micropenis and a healthy penis in newborn babies, but doctors are getting better at diagnosing this condition early and treating it with hormonal therapy prior to puberty. Although some less-endowed adults may take this to mean that testosterone therapy will help them gain a few inches, Brahmbhatt stresses that this is only an effective course of treatment during childhood, and only for children with a micropenis diagnosis. 

It’s important to note that micropenises are relatively rare, and a majority of men aren’t satisfied with their penis size, studies suggest. Such dissatisfaction has been linked with poor sexual health and low self-esteem. Yet there’s no evidence that having a small penis means anything for a man’s sex drive and fertility, unless there’s an underlying hormonal problem. (Also, guys, there are ways to work around a small penis in the bedroom).

The best way to forestall such issues of self-consciousness is to talk about healthy, normal penises with your kids. Brahmbhatt, who is a father of three, acknowledges this is no easy task. But it’s crucial. If kids don’t hear about normal anatomy from you, they’re going to get their information from less reliable sources.

“When they start to explore, they’re probably going to go to porn. And what they’re going to see is not the norm,” Brahmbhatt says. “Discussing it may alleviate some of the stress and anxiety they’ll have, but most parents don’t.”