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Comment author: James_Miller 29 March 2015 09:36:29PM 1 point [-]

Kalos kagathos. I don't this is merely a perception bias.

Comment author: V_V 30 March 2015 12:23:35AM 0 points [-]

Both a true correlation and a perception bias may be present, but it would be difficult to distinguish them without using standardized tests.
Correlations between attractiveness and academic performance or criminal record could be confounded by the perception bias, we would need something like IQ or SAT to have a fair estimate.

Comment author: komponisto 29 March 2015 07:56:20PM 6 points [-]

The thing that puzzles me here is why Knox was ever prosecuted at all. The prosecution had Guede.

The answer is simple and banal: they didn't get Guede until after they had already decided Knox and Sollecito were guilty. Not prosecuting Knox and Sollecito would have required them not only revise to previous beliefs in which they had become psychologically invested, but also to retract previous public pronouncements -- in short, to admit they had been wrong.

From the inside of their minds, no doubt, Knox and Sollecito just felt so suspicious, in the early days of the case before the physical evidence came in and they were relying on behavior to form hypotheses . It's also likely that they were irrationally angry at Knox because of the false implication of Patrick Lumumba that they coerced out of her, and that this anger and frustration at the failure of their own hypothesis morphed into a sense that Knox was an evil vixen.

Comment author: V_V 29 March 2015 09:23:51PM 1 point [-]

Looks like a textbook case of Anchoring.

Comment author: James_Miller 29 March 2015 06:50:08PM 3 points [-]

Working Paper Ugly Criminals "Using data from three waves of Add Health we find that being very attractive reduces a young adult's (ages 18-26) propensity for criminal activity and being unattractive increases it for a number of crimes, ranging from burglary to selling drugs. A variety of tests demonstrate that this result is not because beauty is acting as a proxy for socio-economic status. Being very attractive is also positively associated adult vocabulary test scores, which suggests the possibility that beauty may have an impact on human capital formation. We demonstrate that, especially for females, holding constant current beauty, high school beauty (pre-labor market beauty) has a separate impact on crime, and that high school beauty is correlated with variables that gauge various aspects of high school experience, such as GPA, suspension or having being expelled from school, and problems with teachers."

More generally: Good human traits are almost always positively correlated with most other good human traits.

Comment author: V_V 29 March 2015 09:14:43PM 1 point [-]
Comment author: Izeinwinter 29 March 2015 10:46:13AM 0 points [-]

Bad prior. Gang violence is a major murder statistic, but it's pretty far from being "most". Quick googling says: "1 in 6 murders". The most common motive, at 50% is "Argument". So.. men are more likely to escalate those to homocide?

Comment author: V_V 29 March 2015 01:55:56PM 1 point [-]

Bad prior. Gang violence is a major murder statistic, but it's pretty far from being "most". Quick googling says: "1 in 6 murders".

Makes sense.

Comment author: gwern 28 March 2015 10:41:45PM 1 point [-]

No, male-like murder levels would be higher than genderless murder levels.

...and what do you think that implies about whether female murder levels are lower as I claimed?

And, if I understand correctly, most of the excess male murder rate involves gang-related violence, which in this case was pretty clearly not involved.

Yeah, no. Think about that a little bit. (Also, please note the irony of responding to criticism about not conditioning by claiming it would be neutralized by further conditioning.)

Comment author: V_V 29 March 2015 09:48:12AM 0 points [-]

(Also, please note the irony of responding to criticism about not conditioning by claiming it would be neutralized by further conditioning.)

If the updates on different kinds of evidence would likely cancel each other, it is an argument for avoiding conditioning too hard or privileging one kind of evidence while doing informal reasoning.

Comment author: Douglas_Knight 28 March 2015 10:29:00PM 2 points [-]

No, the male murder excess rate is not gang-related. Why would you think so?

Comment author: V_V 29 March 2015 09:44:21AM *  0 points [-]

I was thinking that most murders are gang-related and most gang members are male, but I see that this is disputed. Unfortunately, all the sources I can find seem to take a partisan position in the gun control debate, hence I don't know.

Comment author: gwern 28 March 2015 07:43:19PM *  8 points [-]

That's still base-rate neglect as you are picking and choosing what you want to look at and not conditioning on one of the more relevant variables.

What fraction of the pretty girls who lived with the victims turned out to be murderers? By looking at the genderless conditional probability ('somebody'), you're implying that women like Knox might have male-like murder levels, which is obviously wrong. And to the extent that pretty girls do not have differing patterns of murdering roommates from other women, you're making the exact same mistake, even (it doesn't matter whether you update on pretty girl then roommate or roommate then pretty girl).

Update on both living with the victim and being female and the small probability is bigger but... still small, since the still relatively low probability of a roommate murdering is penalized substantially by being female (female murder rates are like 1/10th male and that's the raw rates, not adjusted for age or SES or race etc). As the top comment says, "Once we take into account that AK and MK aren't in a relationship, AK is female, and there is very strong evidence that someone else committed the murder then I'd agree that the probability drops".

Comment author: V_V 28 March 2015 09:12:07PM *  1 point [-]

By looking at the genderless conditional probability ('somebody'), you're implying that women like Knox might have male-like murder levels, which is obviously wrong.

No, male-like murder levels would be higher than genderless murder levels. And, if I understand correctly, most of the excess male murder rate involves gang-related violence, which in this case was pretty clearly not involved.

Anyway, I agree that if you are doing pure Bayesian inference you have to condition on all kinds of available evidence, including gender, race, social class, nationality, etc. But we can't expect courts to consider this kind of evidence, for good reasons (avoid creating self-fulfilling prophecies and avoid incentivizing crime within certain demographics).

Comment author: Daniel_Burfoot 28 March 2015 07:44:02PM 4 points [-]

I disagree strongly. The fact that she lives with the victim doesn't shield off the effect of her demographic profile on the likelihood she committed the crime.

Let's analyze the problem using Bayes Net terminology. Let A={suspect=Knox's demographic profile}, B={suspect lives with victim} and C={suspect guilty}. Then your claim is that the net is structured as A->B->C, or that the demographic evidence is conditionally independent of guilt given co-habitation. My claim is that the net is structured as A->C<-B; both A and B affect the likelihood of guilt, and in particular A substantially reduces the likelihood of guilt as James_Miller points out (Note that I'm not saying B is irrelevant, obviously this is wrong).

I am very confident in this claim and would wager long odds in favor of it.

Comment author: V_V 28 March 2015 08:59:21PM 0 points [-]

Let's analyze the problem using Bayes Net terminology. Let A={suspect=Knox's demographic profile}, B={suspect lives with victim} and C={suspect guilty}. Then your claim is that the net is structured as A->B->C, or that the demographic evidence is conditionally independent of guilt given co-habitation.

No.

My claim is that the net is structured as A->C<-B; both A and B affect the likelihood of guilt, and in particular A substantially reduces the likelihood of guilt as James_Miller points out (Note that I'm not saying B is irrelevant, obviously this is wrong).

I agree.

Comment author: James_Miller 28 March 2015 03:28:34PM 7 points [-]

This makes for a good case study in base rate neglect. Girls like Amanda Knox almost never commit the type of murder she was accused of, so unless you have extremely compelling evidence you should think she is very likely innocent. Yet would we want our justice system to grant a higher presumption of innocence to pretty, smart, young women, then, say, lower class men?

Comment author: V_V 28 March 2015 04:13:15PM *  3 points [-]

No. This was already discussed here.

If she had been randomly sampled from the general population then the prior probability would have been exceedingly small, but she wasn't randomly sampled, she was investigated because she lived with the victim. When someone is murdered, there is a high probability that the perpetrator is somebody in a close or frequent relationship with them.

Realistically, the prior probability of Knox being a perpetrator would be around 0.01, the same for Sollecito, with high positive correlation between them.

Comment author: V_V 28 March 2015 10:47:14AM *  20 points [-]

It seems that the Italian judicial system is indeed applying Bayesian inference... using a Monte Carlo approximation :)

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