Penis size is very much a product of genetics. But what determines penis size, 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. So, the question is, what actually determines penis size? Well, that’s largely the province of the Y chromosome. Studies suggest, however, 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. Around the eight-week mark, genitalia starts to develop and differentiate. 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 size genes from one X chromosome, and one from the other.
While 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 penis size. 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, causing a condition known as micropenis. 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 low self-esteem and poor sexual health. 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 because, if they 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.”