so far in the course are human universals, what everybody

shares. So, we've been talking about

language, about rationality, about perception,

about the emotions, about universals of

development, and we've been talking about what people share.

But honestly, what a lot of us are very

interested in is why we're different and the nature of

these differences and the explanation for them.

And that's what we'll turn to today.

So first, we'll discuss how are people different,

different theories about what makes you different in a

psychological way from the person sitting next to you,

and then we'll review different theories about why people are

different. And this is the class which is

going to bother the most people. It's not dualism.

It's not evolution. It's this because the

scientific findings on human psychological differences are,

to many of us, shocking and unbelievable.

And I will just try to persuade you to take them seriously.

Okay. So, how are people different?

Well, there's all sorts of ways. Your sexual identity--It is at

the core of your being for almost all of us whether you're

male or female. How we refer to you in

language, what pronoun we use, is indexed on how we--on

your--on how--whether you're male or female and related to

that though imperfectly is your sexual orientation,

who you're attracted to. The question of why some of us

think of ourselves as males and others as females,

and the question of why some of us would ideally want to have

sex with males, others with females,

others with both, and then a few others who have

harder to define desires, is such a good question that

we're going to talk about it after spring break while all the

sexual desire has been spent and you could focus on [laughter]

on a scientific discussion of this--not that I recommend you

do that on spring break. How happy are you?

This is also such a good topic it's going to get its own class.

The very last class of the semester is devoted to happiness

and the question of what makes people happy,

what makes people unhappy, and what makes people differ in

their happiness. If I asked you to rank how

happy you are from a scale of 1 to 10, the numbers would differ

across this room. And there's different theories

as to why. Your success and failure in

life--This is somewhat interesting because you could

study this in more or less objective ways.

We don't have to ask people. We could look at your

relationships, how they begin,

how they end, your job satisfaction.

We could look at your criminal records.

Some of you are going to see time.

Most will not. Some of you will get into

little troubles all through your life.

Some of you already have seen the inside of a police station,

possibly a lineup. Others couldn't go near such a

thing. What determines that?

And at the root of all human differences are two main

factors. And so, I want to talk about

the two main interesting factors.

One is personality. The other is intelligence.

And this is what--These are the differences I'll talk about

today first from the standpoint of how do we characterize them,

how do we explain them, and then from the standpoint of

why these differences exist in the first place.

One way to characterize personality is in terms of

people's style with dealing with--in dealing with the world

and particularly their style with dealing--in dealing with

other people. So, you take a simple character

you know of and you could talk about that person's personality.

You could talk about it in terms of being impulsive,

irresponsible, sometimes lazy,

good-hearted. You could compare that person's

personality with other people's personalities such as my

colleague who gave a talk last class.

He's wonderful. He's responsible and reliable

and very kind [laughter] and different from Homer.

And so, this difference is a difference in personality.

Now, when we talk about personality we're talking about

something else as well. We're talking about a stable

trait across situations and time.

So, if all of a sudden the person next to you kind of

smacks you in the head, you might be angry but we

wouldn't call that "personality" because that's something that's

a result of a situation. We'd all feel that way in that

situation. It's "personality" if you walk

around all the time angry. That'd be a stable trait.

That'd be something you carry around with you and that's what

we mean by personality. Now, how do we scientifically

characterize differences in personality?

And it's a deep question. There's been a lot of attempts

to do so. Any assessment has--Any good

assessment has to satisfy two conditions.

And these are terms which are going to show up all over

psychological research but it's particularly relevant for this

sort of measure. One is "reliability."

Reliability means there is not measurement error.

And one crude way to think about reliability is,

a test is reliable if you test the same person at different

times and you get the same result.

My bathroom scale is reliable if whenever I stand on it,

it gives me more or less the same number.

It's not reliable if it's off by ten pounds in the course of a

day. Similarly, if I give you a

personality test now and it says that you're anxious and

defensive, well--and then give it to you

tomorrow and it says you're calm and open minded,

it's not a reliable test. So, reliable is something you

could trust over time. "Validity" is something

different. Validity is that your test

measures what it's supposed to measure.

So, validity means it's sort of a good test.

Forget about how reliable it is. Does it tap what you're

interested in? So, for example,

suppose I determine your intelligence by the date of your

birth. I figure out what day you were

born and I have a theory that, from that, predicts how smart

you are. That's my intelligence test,

the date of your birth. Maybe people born in January

are the dumbest, people born in December are the

smartest. Is that--I was born on

Christmas Eve. [laughter]

Is that a reliable test? Yes, it's a wonderfully

reliable test. I'll test you today;

I'll test you tomorrow; I'll test you next year;

I'll test you the day you die; I'll get the same IQ score.

Is it a valid test? It's a joke.

It's absolutely not a valid test.

It has nothing to do with intelligence.

But you noticed these are two different things.

Something can be reliable but not valid and something can be

valid and not reliable. Now, there are no shortage of

personality tests. You could get them all over the

place including on the web. So, I took one recently.

I took "which super hero are you?"

[laughter] And it's a series of questions

determining what super hero you are.

You could take this yourself if you want to.

The same web page, by the way, offers you a test

in whether you're "hot" or not. We'll discuss that later.

And when I did this [laughter] it told me I was Batman

[laughter] and "you are dark,

love gadgets, and have vowed to help the

innocent not suffer the pain you have endured."

Now, the honest-- [laughter] Now, to be honest though,

it's neither reliable nor valid.

When I first did the test I came up as "The Incredible

Hulk." I then changed my answers a bit

and was "Wonder Woman." [laughter]

And finally, out of frustration,

I carefully tailored my answers so I would be Batman.

But the fact that I can do that, well, raises questions

about both the reliability of this measure and its validity.

Here is an example – a real world example.

This is, in black and white form, a version of the Rorschach

test, the Rorschach inkblot test.

How many people have heard of the Rorschach test?

Okay. Is there anybody here who has

actually, in any sort of situation, taken a Rorschach

test? Some people scattered in the

room have taken them. It was originally used only for

psychiatric cases but then became extremely common.

About eighty percent of clinical psychologists claim to

use it and most graduate programs in the American

Psychological Association who are accredited teach it.

Catholic seminaries use it for people who want to join the

seminary. It was invented by a guy named

Herman Rorschach. He devoted his entire life to

the inkblot test. His nickname when he was a

teenager – I am not kidding you – was "Inkblot."

[laughter] And the idea is by looking at

these inkblots and then seeing what somebody says you get great

insights into the nature of their personality,

into what they are. Anybody want to try it?

Come on. Yes.

What do you see? Student:

I see two people holding hands pressed together.

Professor Paul Bloom: Two people holding hands pressed

together. Very good.

Anybody have a different reading?

Yes, in back. Yes.

Yes. Student: Dancing bears.

Professor Paul Bloom: Dancing bears.

Okay. Good.[laughter] Good.

Okay. I got to write your name down--

[laughs][laughter] report you to health-- No.

Dancing bears, very good. Anybody else?

One other. Yes.

Student: A man in a ski mask.

Professor Paul Bloom: A man in a ski mask.

Well, it turns out that there are right answers and wrong

answers to the Rorschach test. According to the test,

and this is from a real Rorschach test,

"it is important to see the blot as two human figures,

usually females or clowns." Good work over there.

"If you don't, it's seen as a sign you have

problems relating to people." [laughter]

If you want to go for "a cave entrance" or "butterfly" or

"vagina," that's also okay. [laughter]

Now, the Rorschach test is transcendently useless.

It has been studied and explored and it is as useless as

throwing dice. It is as useless as tea leaves.

Nonetheless, people love it and it's used

all over the place. It is used for example in child

custody cases. If you have broken up with your

partner and you guys are quarreling over who gets to keep

the kids, you might find yourself in a

shrink's office looking at this. And in fact,

this is why they end up on the web.

There are services. There are people who have been

kind enough to put on the web these inkblots,

including the right answers to them.

But they are worthless as psychological measures.

Can we do better? Well, we probably can.

Gordon Allport did a study where he went through the

dictionary and took all of the traits that he believed to be

related to personality and he got eighteen thousand of them.

But what was interesting was they weren't necessarily

independent traits. So, the traits like "friendly,

sociable, welcoming, warm-hearted" seemed to all tap

the same thing. So, Cattell and many others

tried to narrow it down, tried to ask the question,

"In how many ways are people's personalities different from one

another?" How many parameters of

difference do you need? How many numbers can I give you

that would narrow you in and say what personality you are?

One approach was from Eysenck, who claimed there were just

two. You could be somewhere on the

scale of introverted-extroverted,

and somewhere on the scale of neurotic and stable.

And since there's basically two types of traits with two

settings for each, there are basically four types

of people. Later on he added another trait

which he described as "psychoticism versus