Newborns are too noisy for researchers to concentrate.
Newborn babies are hard to acquire in large enough quantities.
It's impossible to ensure that test is unaffected by discomfort.
New-born infants sleep most of the day.
Perceptual ocular technique
Preferential looking technique
Preventative leading technique
Angle of convergence
They flew into violent rages.
Sudden changes of illumination startled the animals.
They did not blink at a threatening motion towards the face.
They needed to be fed more often.