A realistically rendered skull is more than enough to give some people pause. As we saw in the skull and bat tattoo, the skull is the quintessential symbol of human death. There is simply no other bone in the human skeletal system that is so immediately identifiable.
Perhaps that comes from the fact that the human face is critically important when it comes to personal interaction. So much is conveyed through the face and so much of our attention, from the time we’re born, is spent in recognizing and reading faces. The bare skull, however, is given a bit of a boost in the department of eeriness when it seems animated.

Skull and cherry blossoms tattoo by Greg James.
In this tattoo example, amidst the cherry blossoms and ringed with its own skull necklace, this colorful version still retains its eyes. Bright blue and orange swirls contrast sharply with our expectation for something more sober. The eyes, though without lids or bottoms, seem beady. And the expression in the mouth is enigmatic – read it as menacing, grimacing, or smiling, as you see fit.