So we've been going through
all of the other things that we were assuming are
held constant in order to be moving along
one demand curve. And now let's list a few other. And before I do any more of
them, let's talk about the ones we already talked about. So one, we said that one of
the things we held constant-- let me write this down. So held constant. One of the things that we
held constant to move along one demand curve for the
demand itself to not shift, for the curve to not shift,
is price of related goods. The other thing we assumed
that's being held constant is price expectations
for our good. And now we'll list a couple of
them that are fairly intuitive, but you'll see in
the next few videos that there are often
special cases even to this. So the other thing
that we've been holding constant to stay on one
demand curve is income. And this one is
fairly intuitive. What happens if everyone's
income were to increase? And in real terms, it
were to actually increase. Well then, all of a sudden, they
have more disposable income, maybe to spend on
something like e-books. And so for any
given price point, the demand would increase. And so it would
increase the demand. And once again, when we talk
about increasing demand, we're talking about
shifting the entire curve. We're not talking about a
particular quantity of demand. So income goes up, then
it increases demand. Demand goes up. And remember, when we're talking
about when demand goes up, we're talking about the whole
curve shifting to the right. At any given price
point, we are going to have a larger
quantity demanded. So the whole curve, this whole
demand schedule would change. And likewise if income went
down, demand would go down. And we're going to see in a
future video-- it's actually quite interesting-- that's
not always the case. This is only true
for normal goods. And in a future video we'll
see goods called inferior goods where this is not
necessarily the case. Or by definition for an inferior
good, it would not be the case. Now the other ones that
are somewhat intuitive are population-- once again,
if population goes up, obviously, at any
given price point, more people will want it. So it would shift the
demand curve to the right, or it would increase demand. If population were
to go down, it would decrease
demand, which means shifting the whole
curve to the left. And then the last one we'll
talk about-- and remember, we're holding all of these
things constant in order for demand not to change. The last thing is
just preferences. We're assuming that people's
tastes and preferences don't change while we move along
a specific demand curve. If preferences actually change,
then it will change the curve. So for example, if all of a
sudden, the author of the book is on some very popular talk
show that tells everyone that this is the best book
that was ever written, then preferences
would go up, and that would increase the total demand. At any given price
point, more people will be willing to buy the book. If, on the other hand,
on that same talk show, it turns out that they do an
expose on the author having this sordid past, and the author
plagiarized the whole book, then the demand will go down. The entire curve, regardless of
the price point-- at any given price point, the quantity
demanded will actually go down.