It is extraordinarily rare for a person to be born mute and calculating the exact odds of this occurrence happening is practically impossible. Muteness may result from two conditions: physical muteness, where the person has a problem with the throat or vocal chords rendering them unable to make sounds; and deafness, which the person can make the sounds but not speak.
Occasionally children who are born mute might be physically mute because of genetic factors inherited from their parents and other relatives. Since a child's genetic make-up is dictated by that of their parents, if the parents have genes that can cause physical problems to the throat or vocal chords, the child then could be born unable to create sound. As with other genetic qualities, such as eye color and hair color, the odds of being born mute are higher for children with a family history of difficulty in speaking due to physical problems. However, having family members who became mute after birth does not necessarily mean that the child has increased odds of being born mute. Muteness caused by physical or psychological trauma after birth is not a part of the child’s genetic predisposition. For example, if a child's maternal grandmother became mute due to an adolescent illness, that child does not have a genetic predilection toward muteness.
Since the throat and vocal chords are developed while the child is in the uterus, any substances consumed by the mother that might result in stunted growth in the unborn baby might lead to muteness. This includes any type of potentially harmful substance taken during pregnancy such as alcohol, tobacco and drugs both illegal and, in some cases, prescription. Essentially any substance that could cause developmental harm to a fetus increases the chances of any number of problems in the unborn child's growth, including muteness.
Even with modern medicine and science there are still some factors of childbirth that are simply left to chance. Occasionally a baby doesn't fully develop before birth, which can result in many different physical deformities. these deformaties can range from something as simple as a slightly different shaped finger to something as noticeable as a cleft lip. Although extremely rare, there is an infinitesimal chance that a child could be born mute simply because their throat or vocal chords did not completely develop. The odds are infinitesimal, but still there.
Aside from all physical possibilities of muteness, a child born completely or very nearly deaf in both ears has higher odds of also being unable to speak. While they might be able to laugh, scream, cry and make other noises, they functionally might not be able to make the sounds needed for coherent speech. Humans learn the pronunciation and manners of language by what they hear, so children who lack the ability to hear can also lack the ability to talk. This does not mean they are actually mute, or deaf-mute, but it means that they are effectively mute because of the inability to form the noises they can make into words. Fortunately treatments for deafness, such as cochlear implants, can also help with this form of muteness by allowing the person to hear and therefore slowly learn how to develop speech.
Although not born mute, an estimated one out of 1,000 school age children develop a condition called mutism. Mutism is not a physical condition but rather a psychological and social one where children who otherwise speak normally become completely unable to do so in certain environments or situations, such as at school. Mutism is unrelated to communication or mental disorders, such as stuttering or autism, but is typically a symptom of social anxiety disorder. This inability to communicate can also extend to body language and eye contact. It's important to note that the child remains physically capable of speaking. Children from a family with a history of anxiety have a greater chance to develop mutism.