Perhaps the best-known study of clairvoyance in recent times has been the US government-funded remote viewing project at SRI/SAIC starting through the 1970s through to the mid-1990s.
Current thinking among proponents of clairvoyance circles that most people are born with clairvoyant abilities but then start to suppress them as their childhood training compels them to adhere to what are seen to be acceptable social norms.
Numerous institutes offer training courses that attempt to revive the clairvoyant abilities present in those early years. Another school of thought claims that our so called sixth sense grows when we carry out spiritual practice. With regular spiritual practice done according to basic spiritual principles we are suppose to be able to increase our "spiritual level" and are able to perceive and experience communication with spirits to greater degrees. Clairvoyance is one of the abilities which may be gained by strict discipline.
According to many Taoist and Buddhist related practices, abilities such as Clairvoyance and many other 'supernormal' abilities are signs of spiritual awakening and the rising of human consciousness. Integrating with spiritual and mind expansion is breathing meditation. The vast majority of people only normally use one third of their brains and one third of their lungs at any time.
In Taoist and Buddhist thought this is not a coincidence. By expanding lung capacity and learning to use the lungs as fully as possible pushing forces around the body and opening the energy channels we also naturally expand the mind and elevate mental consciousness. This is how these supposedly miraculous powers develop, though they are not truly miraculous. They are believed to be latent abilities that everyone possesses but need to work to wake up.
Such abilities in some schools of thought are considered distractions from the true path of Enlightenment however and can lead to the practitioner straying from the true path. The re-discovery of these energetic abilities relies on the activation of the 'Dan Tien' which is the central energy reservoir, just below the belly button, that flows through out the body. When the practitioner learns to 'turn' it and move it as if it were a 5th limb then chi can begin to be pushed around the body. 
The Dan Tien is believed to be as strong as a baby but quickly slows to a crawl as one gets older. A major part of the Taoist and Buddhist practice is learning to activate the Dan Tien once again.
This may also explain why such abilities are supposedly stronger as a child and quickly disappear as one ages but can be awakened again at any time by the proper practice of arts such as Nei Gong and Qi Gong to expand the mind and spirit. There are many abilities that can be developed in this way - Telepathy, Prediction, Astral Traveling, Pyrokinesis, Telekinesis, Levitation, Energetic Healing and Empty Force to name a few.