Monday, October 02, 2006

Social Networks and AIDS

I don't usually follow up on a post with another post explaining it, but these are very difficult questions, and I want to share a thought I have.

Talking about the Surgeon General's comment I mentioned that a disease like AIDS moves along a social network. By that I mean a particular thing, there is a science of social networks, related to graph theory in mathematics but applied to sociology and usually practiced by physicists, go figure. Generally, you imagine a graph, where each person is a node, say a little circle, and there are lines connecting various ones of them. Not every node is connected to every other, you typically have dense regions -- clusters -- with sparse connections between them.

You've heard it said that everyone in the world is connected by seven or less steps to everyone else -- the "six degrees of separation" effect. Well, now you're talking about social networks. Who's connected to who, what links you'd follow to get from you to Kevin Bacon. More to the point, you've probably heard it said that when you have sex with somebody, you're having sex with everybody they ever had sex with, and so on, moving out through the social network.

Some diseases are just distributed randomly in the population. Somebody gets the flu, they cough and sneeze, the virus goes into the air, and people nearby are exposed to it. There is some probability of them actually getting sick, but there is no requirement that they actually know the person they caught it from. Some proportion are sick, and that is a parameter of the population; some have recovered already, some have not caught the disease yet, some have died ... these are just parameters that can be entered into a formula or model to make predictions.

On the train-ride home I started thinking about this. After seeing some of the comments on that last post, I realized that some people don't have this concept. So I Googled [AIDS epidemic "social network"]. The first hit was about drug users, so I clicked on the second one.

It's a pdf file of a scientific paper by a lady named E. Ann Stanley, published in a journal called Connections, which is put out by ISNSA, the International Network for Social Network Analysis. She used to work at Los Alamos National Laboratory, then at Iowa State University, and now she lists her affiliation as "Bend, Oregon." I don't know her, but I do know a couple of people who have co-authored with her, and I see that her publications look very good. She was involved in some of the early modeling of the AIDS epidemic, even back into the Eighties.

This 2006 paper is really just a kind of introduction to social networks, drawing upon other more technical papers that the author and others have published. It's called Social Networks and Mathematical Modeling.

This blog is not the place for technical mathematical papers, but let me copy and paste a few paragraphs from this one for you, because it's right on the money.
Mathematical modeling studies have shown that the AIDS epidemic is very sensitive to the human behaviors that spread HIV, including: the amount of risky behavior; the manner in which that risky behavior is distributed in the population; and the social network structures within which people practice those risky behaviors. In fact, these models have shown that if we do not understand all three of these factors, then we cannot hope to predict and control the spread of HIV and other sexually transmitted diseases(Hyman and Stanley, 1988, 1994; Stanley, et al., 1991).

One of the earliest indications that this was so occurred in the mid-1980's. At that time, the US Centers for Disease Control was predicting that the AIDS epidemic was already dying out. They predicted this based upon the fact that the epidemic was growing more slowly than exponentially. With infections that are transmitted via casual contacts, such as measles or the common cold, case data which grow less than exponentially does indeed indicate that an outbreak is peaking and will soon be on the decline. Early mathematical models of the AIDS epidemic also demonstrated exponential growth, followed by slowing towards a peak and then decline (Anderson, et al., 1986)

See, they were thinking it was like the flu, and the only thing you had to know was how many people have it. But AIDS is different. Every contagion event occurs intimately between two people, the disease passes from node to node through the network; you don't catch it from doorknobs, you catch it from somebody you know. And the population-level effects of these two kinds of processes look different.

Excuse me, but I love this stuff.

She continues:
However, while these early models did account for both sexual activity levels and the distribution of those activity levels in the population at risk, they did not account for social network structure. Instead, they assumed random mixing. Simply adding the fact that people who are high risk tend to associate more often with others of high risk, and similarly for those of low risk, to our model, along with data on the distribution of partner acquisition rates from various studies in homosexual populations, showed that the epidemic should be growing cubically in time. Reanalysis of the CDC data showed that this was indeed the case (Hyman and Stanley, 1988, Colgate, et al., 1989)

So the traditional epidemiologists were predicting that the epidemic was dying down, but these guys' model predicted that it was actually growing. Remember, AIDS has a long incubation rate, and people are infectious before they have symptoms, so there can be hidden propagation going on.

When she says "the distribution of partner acquisition rates" she means they look at how many people have lots of partners, how many have kind of a lot, how many have only a few, etc. That histogram will look different for different populations, I'd be sure, and it's an important thing to put into a predictive model. You'd also know how many of the Highs know other Highs, how many Highs the Lows know and vice versa, and so on.
This modeling effort showed that the epidemic was decidedly not dying out. This had important policy implications, since many were saying that, since the epidemic was dying out, it would go away on its own, and it wasn't worth spending a lot of money to control it.
Another important feature of the epidemic that biased mixing models captured was the fact that it was primarily the highest risk individuals who were infected first, followed by the next-highest risk individuals, and so on, whereas the random mixing models had most of their early infections in the large low risk groups.

See how cool this stuff is? If it's the flu, then anybody who rides a bus or train is going to be exposed to it: the "random mixing model." In the "biased mixing model," their model, there is more structure to the spread of the disease.
Modeling HIV spread in age and sex- structured populations pointed out another way in which network structures affect the spread of sexually transmitted diseases. Since female partners tend to be younger than their male partners, women tend to become infected at younger ages than men, with the difference in age being societally-determined, and similar to the difference in age at marriage. But more than that, the way that society is structured, ie the network patterns and norms, strongly influences the speed and pattern of spread. Such questions as who goes to brothels, how many wives and concubines men have, and who gets to have multiple female partners, all affect the spread of disease via sex (Stanley, et al., 1991). Collection and analysis of network data have highlighted some of these factors, showing that clustering of heavily interacting individuals into local risk-taking networks, is an important factor in the spread of disease (Rothenberg et al., 2005 and 2005).

Some of this paper is just instructional and technical and not too relevant to this discussion.

The point is this. As soon as I posted that last comment we had a guy in the comments asserting that the epidemic targets gays because they're promiscuous. That's not what this model says. This model says that you catch the disease from somebody you know, and gay people know each other. And guess what: when you ostracise a subpopulation they tend to cluster together more densely, provide the context for something like this kind of epidemic to happen.

Neat little paper -- not too hard, either. I say, click on the link and see how you like that way of looking at contagion. (It applies to beliefs and attitudes, too.)

11 Comments:

Anonymous Anonymous said...

"This model says that you catch the disease from somebody you know, and gay people know each other."

Well, this disesase is caught by someone you not just know but know carnally. Unfortunately, in a circle of gay acquaintences, it's not unusual for most- or all- of them to have had sex with one another. It's the casualness of it.

But anonoymous promiscuity would be even more efficient and it's well-known that it's widespread in the gay community. This was discussed in a paper you recently linked about rates of new infections.

"And guess what: when you ostracise a subpopulation they tend to cluster together more densely, provide the context for something like this kind of epidemic to happen."

Gays really aren't all that ostracized. People with common interests tend to stick together.

October 02, 2006 11:05 PM  
Blogger JimK said...

Woah, hey, Anon, sounds like you skipped the post about the Heterosexual Agenda. Go back and read that little pamphlet, see how you look to them.

If you're going to throw stereotypes around here, you gotta follow every link, man. It's just as easy for somebody to say stuff about you as for you to say it about them.

JimK

October 03, 2006 6:48 AM  
Anonymous David S. Fishback said...

I think it is pretty obvious that if we establish societal structures that encourage marriage, and work toward cultural norms that encourage monogamy, then we will lessen the spread of STIs (and generally have a happier, more stable society).

Why, then, do people who would agree with that proposition so virulently oppose equal marriage rights for gays and lesbians?

October 03, 2006 10:27 AM  
Anonymous Anonymous said...

"Woah, hey, Anon, sounds like you skipped the post about the Heterosexual Agenda. Go back and read that little pamphlet, see how you look to them."

Didn't read it but suspect this is not how gays look at straights but how they could look at straights, if encouraged. Every gay is a product of a heterosexual relationship, their parents'. They wouldn't desire that the whole world be gay.

"If you're going to throw stereotypes around here, you gotta follow every link, man."

Observations about what behavior leads to or is associated what other types of behavior are appropriate. Obviously, there are individuals of every variety. You notice the commenter used words like "common" and "tendency" rather than "ubiquitous" and "invariable".

The thing is that random and anonymous promiscuity seems to follow logically. If one discards one aspect of traditional morality, what reason is there for them to regard any other aspect of traditional morality?

October 03, 2006 11:29 AM  
Anonymous Anonymous said...

"I think it is pretty obvious that if we establish societal structures that encourage marriage, and work toward cultural norms that encourage monogamy, then we will lessen the spread of STIs (and generally have a happier, more stable society)."

Isn't it pretty obvious that if we establish societal structures that discourage homosexuality, and work toward cultural norms that encourage monogamy, then we will lessen the spread of STIs (and generally have a happier, more stable society)?

October 03, 2006 11:32 AM  
Anonymous Anonymous said...

No, it isn't obvious at all.

We already have societal structures that encourage heterosexual marriage and yet homosexuality continues to exist. Homosexuality has been around since the beginning of time, is found across more than 450 species, and cannot be simply "discouraged" away.

We should embrace people with minority orientation, not marginalize them.

October 03, 2006 12:10 PM  
Anonymous David S. Fishback said...

Anon writes: "The thing is that random and anonymous promiscuity seems to follow logically. If one discards one aspect of traditional morality, what reason is there for them to regard any other aspect of traditional morality?"

This has always been the argument against the exercise of freedom. It assumes that people are so fundamentally venal that they need a set of rules to prevent them from acting venally; that it matters less what the rules are (some rules may be utterly insupportable), than that whatever rules there are are followed. And that a decision to cast aside one rule -- however irrational -- inexorably leads to the collapse of the entire moral framework.

That was the argument of the rulers of Europe at the time of the American Revolution. Our founders rejected that approach. As products of the Enlightenment, our founders believed that people could be trusted to make wise decisions about what traditions to follow and what traditions to discard.

So, over time, we have rejected the traditional morality of subjugation of women, and the holding of slaves. And many other things, once deemed "moral," as well. Should we have stayed with that "traditional morality" for fear of throwing the baby out with the bathwater?

Yes, freedom can be scary. Perhaps sometimes we choose improvidently. (Assuming a fair vote in Ohio in 2004, I think we, as a nation, chose improvidently in 2004.) But the principle upon which the United States was built, and upon which I believe it must continue to rest, is that the orthodoxy of one age is not necessarily the wisdom of the next. The Golden Rule is a fixed lodestar. The rest, we can be trusted to figure out, within a Constitution that protects both individual rights and a republican form of government.

October 03, 2006 4:22 PM  
Anonymous Anonymous said...

I wasn't suggesting any legislation, David. I don't think gay activity or promiscuity should outlawed. Still, don't the arguments for not disregarding the moral implications of one mirror the other? What's the difference?

October 03, 2006 5:05 PM  
Anonymous David S. Fishback said...

Anon,

My responses here go to the issue of what we consider immoral and what we do not consider immoral. If one considers homosexual activity, as opposed to heterosexual activity, as immoral per se, then I suggest that such a person examine WHY he or she believes that all homosexual activity is immoral. And that may open up the question of HOW we decide something is or is not immoral.

October 04, 2006 6:29 AM  
Anonymous Anonymous said...

I agree, David. Parts of your comments seem to be about legalities, though, and I think that's a seperate issue.

Isn't Kant the one who said something like:

Act only on that maxim which you consistently will that everyone should act on?

October 04, 2006 8:59 AM  
Anonymous Anonymous said...

Anonymous, I have been reading your posts, and suffice to say, it doesn't sound like you are the kind of guy who will be persuaded from your rigid outlook at this subject. My question is, have you really analyzed your motives? I mean, why are you spending so much time proselytizing the Gay and Lesbian population, and why haven't you been capable of evolving a set of skills that would allow you to see things rationally? The point of the homosexual 'agenda' as you approach it, is that one MAJOR part of the structure of society has FAILED us. It has put us into a very bad situation. On the one hand, we are raised from children (boys, in particular) to worship a Jesus that is always portrayed as attractive, young, virile, and bearded. In certain evangelical churches, such as the one I was raised in, we were actually told that after the rapture (a concept invented in the 18th century, BTW) that we would all go to heaven to become the "bride of christ!" So, as a boy, imagine how confusing it was to hear that I was meant to become intimately involved with a god some day. That was all well and good until we get to be around 7, 8, maybe 9 years old. Yeah, about the time we discover that there is NO SANTA CLAUS! our faith is shattered by the very people who we've trusted our whole lives. Yes, you continue to expect us to believe that there is a Jesus? As grow into our teens, we're constantly looking for somebody VERY REAL to replace that original ideal, that original god we thought we were supposed to belong to. But men are not gods; far from it. They continue to disappoint, to betray, to do internal damage...and you expect US to continue to believe what you say about society? We left your society to build our OWN society, to separate ourselves from your prying eyes, to make sure you don't get the pleasure of watching as we do what we feel we must to make a divine connection. So, have you finished thinking about WHY you are so obsessed with your proselytizing? I would like to suggest that maybe you look into those dark corners of your own soul and make a choice: do you want to keep forcing religion down our throats, and then ask us why we can't appreciate society's structures? STD's are the furthest thing from our minds when we are trying to find love. And, like in my case, my parents were far too afraid to even discuss STD's with me, primarily because they had NO IDEA what they were themselves. And before you go on to belittle me for not doing what my parents wanted me to do, here's a little tidbit for ya: My Dad, the evangelist, is now a card carrying homosexual himself. Turns out he ALWAYS WAS! So, the reason we don't respect society's structure is because society's structure FAILED US PITIFULLY LONG BEFORE WE BECAME ADULTS!

I mean all of the above in the nicest possible way.

October 08, 2012 1:28 AM  

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