On the dyno: Will a passenger make a difference?

[FiveFreshFish]

When doing a dyno run, will a passenger help, hurt, or make no difference to the results?

This is the thread and video that got me thinking about this.
http://forums.beyond.ca/showthread.php?threadid=71012

[iceburns288]

Can provide extra traction for the car so it doesn't slip on the rollers.

[Phuqu]

For the most part no.

Heat soak on the engine would play a bigger role than extra weight on a non-moving car.

[Aleks]

What if you're on a mustang dyno and you actually load the rollers for the weight of your vehicle? Does that throw the numbers off a bit?

[DEATH2000]

no, the extra wieght shouldnt affect it.