The head position in the deadlift is somewhat of a contentious talking point in lifting circles. 
Some say “fully extend “, some say “ look down”. So who’s right?

Well that depends, the average human head weighs 10-11 lbs and has a great potential for influencing the position of our center of mass.

With that being considered we must not adopt one single head position through the duration of the lift ( head down, or head back ). But rather, we should consider having a dynamic head position, as our center of mass will deviated throughout the lift.

Most lifters could usually benefit from a slightly retracted head position and slight extension of the cervical spine off the floor.

And a slightly deep neck flexed position or “chin-tucked” position at lock-out. This keeps the head centered over the mid-foot and allows for an easier lockout in the knees. 
Hyperextending the head and neck at lock-out often leads to soft knees as your body attempts to stay balanced.

The big takeaway should be that there is no one perfect head position in the deadlift, and lifters should consider a dynamic head position that best compliments their relative proportions in each part of the lift.