monotonic preferences

[EconGuy]

How do you determine weakly monotonic preferences vs strong monotonic preferences? Specificaly Question 1 on the summer 2015 midterm

 Which of the following types of preferences is weakly monotonic, but not strongly monotonic? 

 (a) Preferences represented by a Cobb-Douglas utility function 

(b) Preferences in which one commodity is a good and the other is a neutral 

 (c) Perfect substitutes in which both commodities are goods 

 (d) Preferences represented by a quasi-linear utility function

 (e) None of the above Answers 

(a), (c), and (d) are all strongly monotonic, since increasing the quantity of either good will increase utility. Answer (b) is weakly monotonic, because increasing the quantity of the neutral will not increase utility, but increasing the quantity of both commodities simultaneously will increase utility.

The explanation is not clear to me

[Taylor Stoltzfus]

Hi EconGuy,

This is an old forum. Please use the one under "forum" next time. 

Strongly monotonic: I need more of just one of the goods to be better off

Weakly monotonic: I need more of both goods to be better off

For a perfect complements problem, we need more of both goods to ensure that we are better off, so weakly monotonic 

For a perfect substitutes problem, we need more of only one of the goods to be better off, so strongly monotonic

Best,

Taylor