Chapter 10: The Rational Consumer

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Welcome to the Deep Dive, the show where we take a whole stack of sources and, well, boil them down to what you really need to know.

Glad to be here.

So today we're tackling something that seems pretty basic on the surface, but it's actually fundamental to understanding why people buy what they buy.

When is more?

Just too much?

It's a great starting point.

Let's think about a real world thing.

Picture this.

You're at the Happy Family Chinese Buffet, maybe in Humble, Texas, maybe somewhere else.

It's say $11 .99, all you can eat.

Okay, yeah.

Crab legs, egg rolls.

Exactly.

So if it's truly all you can eat what stops you or anyone from just parking there and eating, I don't know, 10 plates of crab legs, why does the police go bankrupt?

Right.

You pay the entry fee and then, well, nothing's stopping you physically, I guess, until you feel sick.

But most people don't push it that far.

There's a point where you just stop.

What's going on there, economically speaking, is it about hitting some limit?

You're on to it.

That feeling, that decision to stop, that's what we're digging into today.

It's central to this idea of the rational consumer in economics.

We're pulling from Krugman and Wells' microeconomics, specifically Chapter 10, The Rational Consumer, to give you a solid handle on this for your courses, for your exams.

Okay.

So our mission today is to really explore how you, as this rational consumer, make choices, given what you like your tastes and what you can actually afford your budget.

Yeah.

We'll unpack concepts like utility,

budget constraints, how you use marginal thinking to make the best choice, and how all these individual decisions add up to explain the famous law of demand.

The goal is really to give you that shortcut, get you informed, maybe share a few surprising examples, and definitely offer some practical tips for your studies.

Okay.

Let's dive in.

First concept,

utility.

Utility is basically the economist's word for satisfaction or happiness you get from consuming goods and services.

Right.

It's subjective, isn't it?

Like, my satisfaction from coffee is probably different from yours.

Totally subjective.

We can't actually stick a probe in someone's head and measure happiness units,

but assuming people try to maximize this utility helps us model their behavior.

We use a hypothetical unit called utils just to quantify it, to make the models work.

So all the stuff you consume, food, clothes, streaming services, whatever that whole collection is, your consumption bundle.

Yep.

And the relationship between that specific bundle and how many utils of satisfaction it gives you, that's your utility function.

And like you said, it's deeply personal.

Someone might get massive utility from, I don't know, 20 egg rolls while someone else is done after three.

It's all about individual taste.

So let's think about Cassie and her egg rolls from the textbook example.

As she eats more, her total utility, her overall satisfaction goes up.

Right.

But it doesn't go up consistently.

Imagine a graph.

Total utility rises, but the curve gets flatter as she eats more egg rolls.

The first one gives a big boost, the second a bit less, the third even less.

And eventually, if she keeps going, say, to the ninth egg roll in the example, her total utility actually starts to go down.

She feels worse.

Precisely.

And a rational person, someone trying to maximize their satisfaction,

wouldn't eat that ninth egg roll.

They'd stop before the point where more makes them less happy.

So the key isn't just total satisfaction.

It's about the change from one more unit.

Exactly.

That change is called marginal utility.

It's the additional utils you get from consuming one more unit of something.

So if Cassie's first egg roll gives her 15 utils and her second as 12 utils, the marginal utility of that second egg roll is 12.

And this leads us to a really core idea, right?

The principle of diminishing marginal utility.

That's the one.

It's fundamental.

It just means that each additional unit of a good you consume adds less to your total satisfaction than the previous one.

The more you have of something, the less you value getting one more.

Like the first slice of pizza is amazing, the tenth slice, hey, it's just pizza.

Perfect example.

You're getting closer to being full or satiated, so the extra enjoyment diminishes.

Even if it's free, like at that buffet, you stop when the marginal utility of the next bite is zero, or even negative when it starts making you feel worse.

It's fascinating how this plays out.

You mentioned the salmon example.

Oh yeah, it's a great illustration.

Back in colonial America, salmon was so incredibly abundant in rivers that, believe it or not, some servants had contracts saying they wouldn't be forced to eat it more than a few times a week.

Wow.

So high supply meant low marginal utility per serving.

People just got sick of it.

Exactly.

Then fast forward to the 1980s.

Pollution, overfishing, wild salmon became scarce and expensive.

Suddenly it's a luxury good.

One serving felt special, so its marginal utility was really high.

But then aquaculture fish farming takes off.

You're right.

Salmon becomes plentiful again, prices drop, and it's back to being a common item.

Its luxury status, its perceived value, changed directly with its availability, and therefore its marginal utility to the average consumer.

So the big takeaway for students here is that rational consumers are always thinking, even subconsciously, about the margin.

How much extra satisfaction will this next unit give me?

You stop when that extra bit isn't worth it, or makes things worse.

That's the core of utility maximization right there.

Okay,

so diminishing marginal utility explains why we stop even when something is free, like at the buffet.

But most things aren't free.

How does money, or the lack of it, fit in?

Ah, yes.

Welcome to the real world of limits.

That's where budget constraints come in.

We almost always face a limited income.

Which means buying more of one thing forces you to buy less of something else.

Opportunity costs.

Exactly.

Your budget constraint is simply the rule that what you spend can't exceed your income.

And all the combinations of goods you can afford within that income.

Those are your consumption possibilities.

Let's make this concrete with Sammy again.

He's got $20 a week, egg rolls are $4, cokes are $2.

He wants to get the most happiness possible for his $20.

Okay, so we can draw his budget line.

Think of a graph again.

Egg rolls on one axis, cokes on the other.

If he spends all $20 on cokes, $2 each, how many can he buy?

$20 divided by $2, that's 10 cokes.

Right.

And if he spends all $20 on egg rolls, $4 each.

$20 divided by $4, five egg rolls.

Perfect.

So his budget line is a straight line connecting the point, zero egg rolls, 10 cokes, and the point, five egg rolls, zero cokes.

Any point on that line represents a combination of egg rolls and cokes that costs exactly $20.

Like point C in the book, which was two egg rolls and six cokes.

Let's check.

Two times $4 is $8, plus six times $2 is $12.

Yep, $20 total.

So he spent his whole budget.

Any point inside the line below it is affordable, but he'd have money left over.

Assuming more is better, he wouldn't choose those.

And anything outside the line above it?

Simply unaffordable with his $20.

So the rational consumer, wanting the most utility, will always pick a bundle on the budget line.

The challenge then is finding the specific point on that line that gives him the absolute maximum total utility.

That's the optimal consumption bundle.

Right.

We'd look at Sammy's utility function, how many utils he gets from different numbers of egg rolls and cokes.

Then we'd calculate the total utility for each affordable bundle on his budget line.

And in the example, that optimal point, the peak of his satisfaction given his budget, turns out to be that bundle C, two egg rolls and six cokes.

That's where he gets the most happiness for his $20.

Exactly.

It's the combination on the budget line that reaches the highest possible level of total utility.

And it's important to remember this idea of a budget isn't just about money, is it?

Not at all.

Think about limited closet space that's a budget constraint on clothes.

Limited time in a day that's a budget for activities.

Or like those diet programs, Weight Watchers is a classic example.

You have a daily points budget, and different foods have different point prices.

You have to choose how to spend your points to feel satisfied without going over.

It's the same economic logic, limited resources, choices, trade -offs.

This also ties into real -world trends, like that great condiment craze example.

Oh, absolutely.

Think about the grocery store aisle now versus, say, 30 years ago.

Ketchup, mustard, maybe relish.

That was kind of it.

Now it's insane.

Sriracha, gochujang, chipotle, aioli, dozens of artisanal musters.

What happened?

Two things, mainly.

One, tastes changed.

People got exposed to more global cuisines, became more adventurous.

Two, budgets changed.

Rising incomes for many people meant they had more disposable cash to spend on something a little fancier than basic yellow mustard.

So shifting preferences and increased purchasing power drove that market explosion.

It shows how the optimal bundle changes for people over time.

Definitely.

So the key takeaway, the budget line shows what you can afford, and your optimal choice is always on that line, where you maximize your satisfaction, you're bang for your buck.

Okay, maximizing bang for your buck.

That sounds like we need a more direct way to find that optimal point than just calculating total utility for every single bundle on the line.

That seems tedious.

It would be.

And that's where marginal analysis comes back in, specifically thinking about spending the marginal dollar.

How do you allocate your next dollar to get the biggest utility boost?

Ah, okay.

So we're comparing the options at the edge, at the margin.

Precisely.

We need to calculate the marginal utility per dollar for each good.

This tells you how much additional satisfaction you get from spending one more dollar on that specific good.

And how do you calculate that?

It's pretty straightforward.

You take the marginal utility of consuming one more unit of the good, and you divide it by the price of that one unit.

So it's MU of the good divided by P of the good.

MU good, P good.

Let's go back to Sammy.

Egg rolls are $4, cokes are $2.

Suppose the next egg roll would give him, say, 8 utils of marginal utility.

Okay, so the marginal utility per dollar for that next egg roll would be 8 utils divided by $4, which is 2 utils per dollar.

And suppose the next coke would give him 5 utils of marginal utility.

Then the marginal utility per dollar for that coke is 5 utils divided by $2, which is 2 .5 utils per dollar.

Ah, so spending his next dollar on a coke gives him more bang for his buck 2 .5 utils than spending it on an egg roll, 2 utils.

Exactly.

So if he's trying to maximize utility, he should shift his spending.

Maybe buy fewer egg rolls and more cokes in this situation.

He keeps doing this, comparing the marginal utility per dollar for both goods.

Until when?

When does he stop shifting?

He stops when the marginal utility per dollar is equal for both goods.

That's the optimal consumption roll.

When MU roll, peg roll, MU coke, P coke.

Okay, why equality?

Why does that guarantee maximum utility?

Think about it.

If the MU per dollar was higher for egg rolls than for cokes, Sammy could make himself happier.

He could spend, say, $1 less on cokes, losing a certain amount of utility, and then spend that same dollar on egg rolls, gaining more utility than he lost from the cokes, because the MU per dollar is higher for egg rolls.

His total utility goes up.

Got it.

So he'd keep rearranging his spending like that, until the MU per dollar is the same for both.

At that point, shifting a dollar from one good to the other doesn't increase his total utility anymore.

He squeezes all the happiness he can out of his budget.

That's the logic.

There's no way to rearrange his spending to do better.

The bang per buck is equalized across all the goods he's buying.

This is a really critical point, especially for exams, right?

The rule isn't just that marginal utilities are equal.

Absolutely not.

That's a common mistake.

Yeah.

It's also not where marginal utility equals the price.

It has to be marginal utility per dollar spent, because that takes the cost, the budget constraint, into account.

That makes sense.

It also explains some things that might seem weird otherwise, like people paying more for those little 100 -calorie snack packs.

All right, it looks irrational you're paying more per ounce, but people are rationally buying something else.

Convenience, portion control, hand -to -mouth restraint, as the book calls it.

They're paying extra for the utility of not having a giant open bag tempting them.

So it's a broader definition of utility.

The convenience or self -control is part of the value.

Exactly.

So remember this formula, this principle.

MU1P1 equals MU2P2.

That's your analytical tool for finding the optimal consumption bundle.

Equalize the bang for your buck.

Okay, we've got individual consumers maximizing utility within their budget.

How does all this connect to the bigger picture, to the market demand curve that we always see sloping downwards?

Great question.

These individual optimization decisions are exactly what create the market demand curve.

It comes down to how consumers react when the price of a good changes.

There are two effects to consider.

Let's start with the first one.

The first is the substitution effect.

When the price of a good, let's say, egg rolls, goes up, what happens to its marginal utility per dollar, MUP?

Well, P goes up, so MUP must go down.

The bang for your buck from egg rolls decreases.

Right.

And compared to cokes, which haven't changed price, egg rolls are now relatively more expensive.

So consumers have an incentive to substitute away from the now pricier egg rolls and towards the relatively cheaper cokes.

So price goes up, quantity demanded goes down because people switch to alternatives.

That makes sense.

And for many goods, especially ones that don't take up a huge chunk of your budget, this substitution effect is pretty much the whole story behind the downward sloping demand curve.

But you said there are two effects.

What's the second one?

The second is the income effect.

This becomes more important for goods that represent a significant portion of your spending, like say housing or maybe gasoline if you commute a lot.

How does that work?

When the price of such a major good changes,

it doesn't just make you substitute.

It also effectively changes your overall purchasing power, your real income.

Ah, OK.

So if the price of rent goes way up, I feel poorer overall, even if my salary hasn't changed.

Exactly.

Now think about normal goods.

These are goods you buy more of when your income rises and less of when your income falls.

Most goods are normal.

If the price of a normal good like rent goes up, your real income effectively falls.

And because it's a normal good, that fall in real income makes you want to buy less of it.

This income effect reinforces the substitution effect.

Both push you to consume less rent, making the demand curve slope down even more strongly.

But what if the good isn't normal?

What about inferior goods?

Right.

Inferior goods are those you buy less of as your income rises, like maybe cheap instant noodles if you get richer, you buy steak instead.

So if the price of an inferior good goes up, your real income effectively falls.

Which for an inferior good would make you want to buy more of it.

Exactly.

So for inferior goods, the income effect actually works against the substitution effect.

The substitution effect says buy less because it's relatively pricier, but the income effect says buy more because you feel poorer.

Whoa.

So which effect wins?

Usually the substitution effect is stronger.

So the demand curve for most inferior goods still slows downward.

But there's a theoretical and very rare exception.

Don't tell me the demand curve can slope upwards.

It's called a Giffen good.

This is an inferior good where the income effect is so powerful, it actually overwhelms the substitution effect.

So when the price goes up, the quantity demanded also goes up.

That sounds bizarre.

Does that actually happen?

It's incredibly rare, mostly historical or specific circumstances.

The classic, though debated example, is potatoes during the Irish Famine.

Or perhaps very poor populations relying heavily on a basic staple like rice.

If rice gets more expensive, they might be so much poorer, they can only afford rice and have to cut out everything else, thus buying more rice overall.

But for most goods you'll encounter, assume the demand curve slopes down.

Okay, Giffen goods are the weird exception.

Let's bring this back to something more common.

You mentioned the 2015 gas price drop.

Yeah, that was a perfect real -world example of both effects.

Gas prices plummeted.

First, the income effect.

Because gas is a significant expense for many households, this price drop acted like a tax cut.

People felt richer, they had more purchasing power.

And what did they do with that extra income?

Well, data showed spending increased, especially for lower -income families, on things like fast food and other discretionary items.

That's the income effect feeling richer led to more spending elsewhere.

And the substitution effect?

Gas itself became relatively cheaper compared to everything else.

So people bought more gas, maybe took longer trips, maybe bought less fuel -efficient cars like SUVs, sales of electric vehicles dipped a bit, people substituted towards the now relatively cheaper gasoline.

Both effects were clearly visible.

That really clarifies how they work together.

What about the business case study, McDonald's versus Burger King and the plant -based burgers?

That's a great example of how changing tastes and willingness to pay influence markets.

McDonald's historically focused on value, the dollar menu, appealing to budget -conscious consumers.

But they struggled a bit as chains like Chipotle came in, offering fresher ingredients at higher prices, appealing to different tastes, and frankly, higher incomes.

Right, different optimal bundles for different people.

Exactly.

Then Burger King rolls out the impossible Whopper, a plant -based burger.

Crucially, they priced it higher than their regular Whopper, about a dollar more.

Which seems counterintuitive if you're just thinking price.

But it worked.

It attracted a new demographic.

Younger consumers, Millennials, Gen Z, who were interested in sustainability, health, or just novelty.

Their preferences were different, and they were willing to pay a premium for that perceived value.

So their utility calculation included factors beyond just taste and price, like environmental impact.

Precisely.

It shows how understanding consumer preferences, their utility functions, and their budget constraints, including what they value enough to pay more for, is crucial for businesses.

It shifted demand and influenced the choices of millions.

So understanding substitution and income effects really helps you dissect why demand curves behave as they do, and spot those interesting shifts in the market.

Okay, this has been a really comprehensive look at the rational consumer.

We've gone from figuring out why you stop eating at a buffet.

All the way to understanding how individual choices about utility and budgets,

marginal thinking, and responses to price changes actually build the market demand curves we study.

You should now have a much clearer picture of utility, budget lines, that crucial concept of marginal utility per dollar, and the substitution and income effects.

Definitely a solid foundation for tackling these topics in your course.

So as we wrap up, here's something to think about.

What non -monetary budget constraints are really shaping your choices day to day?

Is it time,

energy,

maybe even just mental bandwidth?

Yeah, and how do your unique tastes, your personal utility function, lead you to an optimal consumption bundle, maybe for how you spend your weekend?

Or choose your classes that looks completely different from someone else's, even if you face the same external constraints?

Lots to chew on there.

From the Last Minute Lecture team, thank you for joining us for this deep dive into microeconomics.

Thanks for listening.