Penis size is, in fact, hereditary. But what determines penis size? Is penis size genetic? 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 exactly straightforward. Penis size, is largely the province of the Y chromosome. But studies also suggest that penis size is 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.”
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. Scientists aren’t sure which parent has more of an influence over a baby’s penis size. But considering genetic brothers can have vastly disparate penis sizes, some experts suspect that there’s more influence from a mothers’ 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.
So 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 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 can lead to a micropenis, which is defined as a penis that’s less than three inches. While it can be hard to distinguish between a micropenis and a healthy penis in newborn babies, 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 dick 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.”