The "Starchild" skull is a curiously deformed human skull that was found in Mexico sometime around 1940. It has attracted an unusual amount of attention due to it being deformed in a way that makes it look like the skull of a "Grey" space alien.
Before I go any further in this I need to note that objectively reporting about this object is made difficult because all of the sources available are by people who are deeply invested in the belief that the skull is of some sort of alien-human hybrid. Personally, I do not share this belief - I do however recognise that the deformities are unusual and noteworthy. However, one cannot solely blame the proponents of the "alien hybrid" idea for the lack of objective information. Due to lack of funding and a reluctance of most scientists to become associated with something that has the taint of the paranormal, the skull has not yet been analysed thoroughly.
The most striking feature of the deformed skull is the greatly increased size of its brain cavity compared to the rest of its features, which are those of a child. Where a normal human skull will only extend a little bit beyond the width of the eye sockets, on this skull the width of the sockets is only about 60% of the maximum skull width. It also exhibits other deformities that are less clearly visible but just as strange. Its foramen magnum, the hole where the spinal cord enters the skull, is shifted forward significantly. It lacks the inner occipital protuberance, which, in normal humans, holds the cerebellum in place. Furthermore, it lacks the inion, the bump at the back of the base of the skull, and it has severely reduced signs of neck muscles. The size of its "chewing muscles" must also have been greatly reduced - this means that it could have supported only a small mandible. It also lacks sinuses. (This can happen as a rare condition in otherwise normal skulls. There are also indication that its bone chemistry is abnormal, as it seems to exhibit a tolerance to acids used to dissolve human bone for DNA testing.)
An image comparing the skull with a normal human one is available at http://www.starchildproject.com/compare-01.html .
Explanations for its Appearance
There are three scientific explanations for the condition of this skull, however there are valid counter-arguments to all.
Explanation One: Skull Binding
In several ancient cultures, such as the Maya, it was tradition to constrain the heads of infants in ways that would cause their skulls to develop abnormally. Typically this resulted in an elongated skull.
Problems with Explanation One
Skull binding results in perfectly flat skull surfaces. However, the surface of this skull is not flat in the affected areas. Also, it is deformed beyond the point where the neck usually begins. Also, this explanation does not account for features like the lack of sinuses.
Explanation Two: Genetic Defect
The skull's deformities could be caused by some form of genetic defect.
Probems with Explanation Two
The skull's appearance does not match any known genetic defect. Also, most such defects result in asymmetric skulls, whereas this skull is perfectly symmetric.
Explanation Three: Congenital Defect
A congenital deformity results from a mis-development in the womb unconnected to any genetic factors. It can account for arbitrary deformations.
Problems with Explanation Three
Congenital deformities usually only affect one area of the skull, for example only the face. The overall shape of this skull could only be explained by several deformities. However, the chances of a child with so many congenital deformities surviving even to be born are pretty much nil.
The popular UFO-ologist explanation for the condition of the skull is that it is an alien-human hybrid - the result of an alien abduction or similar.
Problems with Explanation Four
Well, Occam's Razor does not go well with this explanation at all, for a start. Another problem is that interspecies breeding does not result in anything viable, unless the species are very closely related, like Donkeys and Horses. If the two species are from different planets, this becomes, well, very improbable. A slightly different version of this is that the skull is the result of genetic engineering. If we accept the idea of technologically highly advanced aliens, this is probably possible. But we'd have to accept that first. (The pros and cons of this are an entirely different node topic, and I will not cover them here, lest this writeup become absolutely gigantic.)
The Starchild Project
The Starchild Project is an organisation that is trying to analyse the skull. To their credit, it must be said that they are doing everything they can, given their limited budget, to have the skull analysed thoroughly and scientifically. To their discredit, it must be said that though they are quick to point out holes in pedestrian explanations for the skull's appearance, they do not apply the same critical rigour to their own theory of alien descent.
Most of this information is taken from http://www.starchildproject.com. Some of the conclusions and critical analyses are my own.