law of diminishing returns

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The tendency for a continuing application of effort or skill toward a particular project or goal to decline in effectiveness after a certain level of result has been achieved.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The tendency for a continuing effort toward a particular goal to decline in effectiveness after a certain amount of success has been achieved.
• noun economics A relationship between input and output, such that adding units of any one input (labour, capital etc.) to fixed amounts of the others will yield successively smaller increments of output.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a law affirming that to continue after a certain level of performance has been reached will result in a decline in effectiveness