Adults who lack a certain degree of coordination between the speech center of the brain and the mouth will find it difficult to formulate spoken sentences. This lack of coordination is actual hereditary, so if you have a parent or a relative who stutters, you will have a higher chance of stuttering.
In public speaking situations, if you are not confident in your ability to speak, you will find that your stutter is much more apparent. This is because nervousness further distracts you from being precise in your speech patterns. In any situation where you find yourself particularly nervous, you will probably find that your stuttering is slightly worse than it normally is.
If you had a stutter when you were a child and did not have it properly treated, that stutter might re-emerge later in adulthood. If you did not develop smooth, regular speech patterns at an early age, the problem will come back to haunt you later on in life, even if the stutter appears to get better during your teenage years.
One of the least common but most severe causes of adult stuttering is the lack of proper transmission between the speech center of your brain and the nerves and muscles that control speech. This is known as neurogenic stuttering. This type of stuttering is often caused by a stroke or other brain injury. Neurogenic stuttering is more specifically caused by lesions and abnormalities in the speech section of the brain.