HIV Prevalence in Africa Explained by "Marital Shopping"

If the transmission rate of HIV is low, then how have so many young women on the continent become infected?

There are two facts about HIV that are difficult to reconcile. The first fact is that the transmission rate of the disease is extremely low; the risk of being infected from an person who has the disease through vaginal intercourse is about one in a thousand or 8-12% per-partner-year.* The second fact is that the disease has an extremely high prevalence among heterosexual women in Sub-Saharan Africa; 40% of pregnant women in Botswana and 25% in South Africa are infected with the disease.


The question is then, if the transmission rate is low then how did so many young women become infected?

It may be tempting for those of us in the West to believe that the prevalence of HIV in Africa is related to high rates of promiscuity and commercial sex. But Jeremy Magruder, a young economist at the University of California at Berkeley, is about to publish an extremely compelling argument that shows that African women have become infected with HIV through doing what we all do; shopping for a mate.

In brief, his argument goes like this. Women and men in Africa experience a brief period in their lives while searching for the right marriage partner in which they are in a series of monogamous relationships with high turnover. This searching behavior generates a constant pool of individuals in short-term relationships. Rates of HIV transmission may be low on average, but the probability of transmission from a person who has become recently infected is as much as 10 times higher. So, introduce one recently infected person into this pool of people who are searching for a mate and the whole pool are at risk of being infected.

The paper finds that the introduction of one person per hundred into a searching pool will lead to an HIV prevalence similar to that in Kenya or Tanzania. The introduction of just three infected people per hundred into the searching pool will lead to South Africa's epidemic prevalence rates.

Only a small fraction of sexually active people need to be engaging in risky behaviors in order to create the pandemic despite low transmission rates.

So why are so many women infected with HIV in Sub-Saharan Africa compared to the West, where women engage in the same type of searching behavior?  The difference is that in Western countries a sufficient number of people in the marital searching pool use condoms in short-term relationships.

In my mind, the most important implication of this research is that reducing HIV prevalence in Africa to North American levels does not require a major change in African social norms nor would it require a major medical intervention. It doesn’t even require that 100% of single people searching for a mate use condoms during sex. The economic model suggests that if only 50% of the participants in the marital searching pool use condoms for the first three months of their relationships, the prevalence rates would drop to those comparable to the West.

Health campaigns that strive to convince people that they should use condoms for the rest of their lives, even after marriage, have not been effective. Everyone wants to have sex without a condom eventually. If you know that you won’t be using a condom in six months, and believe that the risk of infection then is the same as it is now, then why use one today?

A public health campaign targeted to young single people encouraging condom use in the first three months of new sexual relationship is bound to have a bigger effect on behavior. If it isn’t promiscuity that is killing women in Africa, but marriage, then this small change in behavior could be what is needed to save the lives of millions of women and children.

* This transmission rate of 8-12% is the African per-person year (PPY) transmission rate. The PPY rate in the US and European is in the 5-10% range.

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New fossils suggest human ancestors evolved in Europe, not Africa

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Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.

A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.

Rethinking humanity's origin story

The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)

As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.

David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.

The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.

Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"

He recently outlined this idea at a conference of the American Association of Physical Anthropologists.

It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.

"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."

Migrating out of Africa

In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.

Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.

The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.

The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.

Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.

Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.

Did we head east or south of Eden?

Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.

Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.