Penis size is very much a product of genetics. But what determines penis size? Is penis size hereditary? As it turns out, these questions — as well as those to questions of where those genes come from and how much the environment plays a role in who is well-endowed and who isn’t — are not entirely straightforward. Penis size, it turns out, is largely the province of the Y chromosome. But studies suggest that penis size is, unsurprisingly, a joint genetic effort between mothers and fathers. 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 Dr. Jamin Brahmbhatt tells Fatherly. “So you can’t entirely blame your mom for your small penis.”
For the first seven weeks of development in the womb, no fetus has a penis. Genitalia starts to develop and differentiate around the eight-week mark. Those given a Y chromosome start to grow a penis. Scientists aren’t sure whether a mother and father’s genetic influence over penis size is 50/50, 60/40, or any other specific ratio. But some experts suspect that there’s more influence from a mothers’ two X chromosomes because true brothers can have vastly different penis sizes. If size was entirely from the Y chromosome, men with the same father would all have essentially the same penis. But if size is largely due to the X chromosome, then it’s possible for one son to inherit penis size genes from one X chromosome, and one from the other.
While, yes, penis size is largely hereditary, environmental factors do play a role. A mother’s exposure to chemicals such as phthalates, as well as drugs and alcohol, can impact the size of a penis. But when a baby is born with a small penis due to environmental factors, his 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 infant boys do not produce enough testosterone on their own. This causes a condition known as micropenis, which is a developed penis that is less than three inches. While it may seem like every baby has a micropenis, doctors are getting better at diagnosing this early and treating it with hormonal therapy, prior to puberty. Though 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 penis size is more often an indicator of future mental health issues than physical ones. A majority of men aren’t satisfied with their penis size, studies suggest, and such dissatisfaction has been linked with poor sexual health and low self-esteem. And 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 it).
The best way to forestall such issues 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.”