the supply curves

Since supply is generally directly proportional to price, supply curves are almost always upwards-sloping.
Occasionally, supply curves do not slope upwards. A well known example is the backward bending supply curve of labour. Generally, as a worker's wage increases, he is willing to supply a greater amount of labor (working more hours), since the higher wage increases the marginal utility of working (and increases the opportunity cost of not working). But when the wage reaches an extremely high amount, the laborer may experience the law of diminishing marginal utility in relation to his salary. The large amount of money he is making will make further money of little value to him. Thus, he will work less and less as the wage increases, choosing instead to spend his time in leisure

2007-03-09 11:08:10 補充：
also see: http://en.wikipedia.org/wiki/Backward_bending_supply_curve_of_labour

yes, in indifference curve approach, 如果個-ve substitution>-ve income effect, derive到個 giffen good,
引申到條upward sloping money income constant demand curve.
將佢反轉睇, 條supply 咪係 downward sloping!!

2007-03-10 11:56:35 補充：
上面打錯左 !!!!應該係 -ve income effect outweigh -ve substitution

i never heard about downwarding Supply Curve.........is that a question in the exam or test?