There are many factors, but in a nutshell, we can say that the most important are how many studs the male bracket is attached to, how many screws are used to attach it, and the kind of screws that are used.
Typically, in North America and/or English-speaking countries such as the UK/Canada/USA/New Zealand/Australia and Ireland, the buildings are constructed with 2 x 4 wood frame construction.
In most cases we attach the male bracket into 2 studs with each receiving 2 x 2 ½ inch pan head wood screws. This is the standard in those countries and therefore the capacity is around 300 lbs, per 24-inch long shelf, measured at a distance of 8 inches from the wall.
The thickness of the shelf does not change the load capacity.
The length however does make a difference. If we are in 3 studs with 6 screws, the load is usually spread over the length of the shelf and therefore the load capacity increases. Though not necessarily exponentially.
Please refer to the maximum load chart done by All North Structural engineers for further data, such as drywall only, concrete, galvanized steel studs etc.
And do remember in this chart, the calculations are calculated 8 inches from the wall with a security factor of 4 x. Our live trials at 8 inches off the wall are just another safety factor.