Trans-species polymorphisms

A trans-species polymorphism. Some genetic polymorphisms are found in distantly related species, having persisted across multiple speciation events. (source)

It is widely known that considerable genetic overlap exists between human populations, even those that are geographically distant from each other and quite different physically. You probably learned in BIO101 that genetic variation is much greater within than between human populations.

It is less widely known that this high degree of genetic overlap also exists between many species that are nonetheless distinct morphologically, physiologically, and behaviorally (Frost, 2011). This is especially so with young sibling species. Such species differ only over a small fraction of the genome—at those genes where a certain variant is adaptive in one species but not in the other. Elsewhere, over most of the genome, the same variant works just fine in both species, either because the gene itself is of little or no value or because certain body functions are pretty much the same in a wide range of organisms.

With time, and reproductive isolation, two sibling species will gradually lose this genetic overlap, as a result of random mutations here and there over the entire genome. The two species will be less and less alike even at “junk genes” of little value.

Even so, some overlap will remain. It’s not just that we see the same gene in distantly related species. We also see the same gene with the same set of alleles—a trans-species polymorphism (Klein et al., 1998). A good example is the ABO blood group system. On the basis of that gene marker, I probably have more in common with certain apes than I do with some of my readers. Such polymorphisms have in fact persisted for millions of years across multiple speciation events.

Until recently, it was believed that trans-species polymorphisms were no more than an oddity. Now, it looks like they may be more common than previously thought: 

[…] we searched for trans-species polymorphisms between humans and chimpanzees using genome-wide resequencing data for 10 western chimpanzees from the PanMap project and 179 humans from the 1000 Genomes Pilot 1 data. […] In addition to the MHC region, we identified over 100 cases, a set significantly enriched for transmembrane glycoproteins, which are often involved in interactions with pathogens. To further rule out the possibility of deep coalescent events by chance, we examined patterns of variation in seven samples of Gorilla gorilla. We discovered 25 cases shared among all three species, which we verified by Sanger sequencing. In a subset, within species diversity levels were unusually high and the tree of haplotypes clustered by allelic type rather than by species, providing definitive evidence for trans-species polymorphisms. (Segurel et al, 2012)

At such genes, variation within species exceeds variation between species … and even between genera.

So just what, then, makes a species a species? The traditional answer is reproductive isolation, and the resulting accumulation of genetic differences over time. Yet this answer seems increasingly problematic. On the one hand, we have cases of living fossils that remain essentially the same over eons of time. Analysis of “junk DNA” would show a steady accumulation of genetic change over those eons, although nothing has changed in  appearance or behavior. A coelacanth today is still a coelacanth after millions and millions of years.

On the other hand, we have cases of sibling species that have emerged in recent times and have become quite different from each other both anatomically and behaviorally. Yet genetic analysis of such species often shows considerable genetic overlap. If we use any of the usual genetic markers (blood groups, enzymes, etc), individuals may not be assignable to a single species with reasonable certainty.

So if genes in general don’t matter, what exactly does? What matters is what matters. Genes for highly adaptive traits matter. Differences you can see matter. Therefore, reproductive isolation in itself is not what makes two populations different; it’s the different ways in which they adapt to different environments.

If a population splits in two with one group moving into one environment and the other moving into another, the two groups will nonetheless continue to look and act similarly as long as their respective environments remain similar (of course, if the two groups are human societies, one of them might create a radically different cultural environment). It is the difference in selection pressures, as a result of differing environments, that will drive them apart … and such differentiation will proceed even if reproductive isolation is still incomplete:

Judging from the number of studies devoted to it, the nature of a reproductive barrier is currently central to the interests of researchers working on speciation. It seems to us, however, that the process of adaptation to the environment is a much more important and interesting part of speciation. The erection of the reproductive barrier may mark the end of speciation, but it tells us little about the process that makes the species differ from one to another, the process that creates biological diversity. (Klein et al., 2007)

References

Frost, P. (2011). Human nature or human natures? Futures, 43, 740–748. http://dx.doi.org/10.1016/j.futures.2011.05.017

Klein, J., A. Sato, S. Nagl, and C. O’hUigin. (1998). Molecular trans-species polymorphism, Annual Review of Ecology and Systematics, 29, 1-21.

 

Klein, J., A. Sato, and N. Nikolaidis. (2007). MHC, TSP, and the origin of species: From immunogenetics to evolutionary genetics, Annual Review of Genetics, 41, 281-304

http://e-groups.unb.br/fm/lmpdc/arquivos/artigos/MARILEN%20QUEIROZ%20MHC%20evolutionary%20genetics.pdf

Segurel, L., E. Leffler, Z. Gao, S. Pfeifer, A. Auton, O. Venn, L. Stevison, A. Venkat, J. L. Kelley, J. Kidd, C. Bustamante, R. Bontrop, M. Hammer, J. Wall, P. Donnelly, G. McVean, & M. Przeworski. (2012). When ancestry runs deep: Trans-species polymorphisms in apes, Annual Meeting of the American Society of Human Genetics, November 6-10

http://www.ashg.org/2012meeting/abstracts/fulltext/f120121882.htm

On paternal age and IQ

Do older fathers have dumber children? For the past millennium, paternal age has been relatively high in Europe west of the Hajnal line. Yet, if anything, mean IQ is higher there than elsewhere. H/T to JayMan (source)

Greg Cochran has been running a series of posts on paternal age and IQ (here, here, and here). His argument is similar to the one he put forward for temperature and IQ:

1.   Because intelligence requires the correct functioning of large numbers of interacting genes, it is more sensitive to random mutations, perhaps more so than other genetically influenced traits.

2.   The higher the mutation rate, the greater the likelihood that something will go wrong. Mean IQ should thus be lower in a population with a higher mutation rate.

3.   As men grow older, they accumulate mutations in the DNA they pass on to their offspring. Children born to older fathers should thus, on average, be more mentally deficient.

4.   Mean IQ should thus be lower in populations where men begin to reproduce at an older age. This is notably the case in highly polygynous populations, such as the ‘female farming’ peoples of sub-Saharan Africa, Papua-New Guinea, and Australia, where women can more easily provide for themselves and their children without male assistance. In such societies, it costs less for a man to take a second wife and, typically, it is the older men who monopolize the women.

This argument is plausible. Polygyny does increase paternal age, as Pierre van den Berghe notes:

In most human societies, males marry later than females, partly because their maturation is slower, partly because older males with more resources take more than their fair share of nubile women. This is especially true in polygynous societies. Typically, the more men are polygynous in a given society, the greater the age difference between husbands and wives. Indeed, given an approximately equal sex ratio, this mean difference in age of marriage between males and females is the main factor making extensive polygyny possible. (van den Berghe, 1979, p. 51)

How large is this sex difference? Among the Nyakyusa of southern Tanzania and northern Malawi, the difference is said to be ten years or more:

[…] there is a difference of ten years or more in the average marriage-age of girls and men, and it is this differential marriage-age which makes polygyny possible. A legal marriage is effected by the transfer of cattle from the groom to the bride’s father, and the system is linked with a late marriage-age for young men and with a privileged position for the older men. Many men have not cattle with which to marry until they are well over 25, while their fathers may be able to afford more than one wife. (Wilson, 1950, p. 112)

In their review of polygyny in sub-Saharan Africa, Pebley and Mbugua (1989), found that polygynous households were headed by men who were, on average, in their late 30s to their early 50s. It is worth remembering that these figures come from the late 20^th century, when male life expectancy was much longer than it had been earlier. Polygynous men were now living to much older ages. This point applies to Cochran’s observation that mean paternal age is 47 in rural Gambia (Cochran, 2012c). Not so long ago, most men died before their mid-forties  ...

It is also worth remembering that marriage is not the same thing as reproduction. In highly polygynous societies, men often begin reproducing before marriage:

The temporary celibacy of young men in polygynous societies is rarely absolute, however. While it often postpones the establishment of a stable pair-bond and the procreation of children, it often does not preclude dalliance with unmarried girls, adultery with younger wives of older men, or the rape or seduction of women conquered in warfare (van den Berghe, 1979, pp. 50-51)

But these are not the main shortcomings of Cochran’s argument. The main one is the existence of another culture area where men typically did not marry until their late 20s or early 30s. This is Western Europe, specifically Europe west of the Hajnal line, where most young men had to postpone marriage until they could inherit a plot of farmland. This pattern is attested in the earliest marriage records, notably those of an English community between 1252 and 1478:

The average age at first marriage in the Lincolnshire Fenland before the Black Death would be 24 years for the woman and 32 years for the man. The wife would die one year before her husband and the marriage would last for about 13 years. The couple could have six children, if their fertility was higher than average, of whom, judging by pedigrees, perhaps three would survive to become adults. After the Black Death the mean age would be 27 for the woman and 32 for the man. The husband would die three years before his wife and the marriage would last about 12 years. Again the couple could have six children, of whom perhaps three would survive to become adult. (Hallam, 1985, p. 66)

This pattern may go even farther back in time. Seccombe (1992, p. 94) cites a 9th-century survey of the Church of St Victor of Marseille, where both men and women appear to have married in their mid to late twenties.

So, for the last millennium, most Western European men began to reproduce at around the same age as their counterparts in sub-Saharan Africa. Over most of that period, they would have ended their reproductive careers at an older age, given the difference in life expectancy.

Yet it is in Western Europe where we find some of the highest mean IQs in the world. One might counter that IQ has been pushed upward by strong selection pressures, notably those associated with gene-culture co-evolution. Undoubtedly. But, then, why cannot gene-culture co-evolution stand alone as a sufficient explanation … not only for Western Europeans but also for sub-Saharan Africans?

References

Cochran, C. (2012a). Gerontocratic polygyny, August 26, West Hunter

http://westhunt.wordpress.com/2012/08/26/gerontocratic-polygyny/

Cochran, C. (2012b). Obvious! Yessss! It was obvious! September 5, West Hunter
http://westhunt.wordpress.com/2012/09/05/obvious-yessss-it-was-obvious

Cochran, C. (2012c).  Gambia, September 17, West Hunter
http://westhunt.wordpress.com/2012/09/17/gambia/

Hallam, H.E. (1985). Age at first marriage and age at death in the Lincolnshire Fenland, 1252-1478, Population Studies, 39, 55-69.

JayMan (2012). More on farming and inheritance systems – Part 1: IQ, June 14, JayMan’s Blog

http://jaymans.wordpress.com/2012/06/14/more-on-farming-and-inheritance-systems-part-i-iq/

Pebley, A. R., and Mbugua, W. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (Ed.), Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe, London: Verso.

van den Berghe, P.L. (1979). Human Family Systems. An Evolutionary View. New York: Elsevier.

Wilson, M. (1950). Nyakyusa kinship, in Radcliffe-Brown, A.R., and Forde, D. (Eds). African Systems of Kinship and Marriage. (pp. 111-139), London: Oxford University Press.

 

East Germans are getting a lot smarter ... Really?

Unemployment rates in the West and East of Germany (source)
Did East German conscripts become smarter because East Germans were becoming smarter? Or were smarter individuals increasingly seeing the army as an alternative to unemployment?

In a recent study of conscripts entering the German army, Roivainen (2012) has found that the mean IQ of East German conscripts jumped eight percentage points between 1990 and 2006. That’s a huge increase! And a fast one! Far too fast to be due to genetic evolution. Sixteen years isn’t even one generation. The only possible cause seems to be a better environment, such as through improvements to the standard of living, education, and nutrition.

Specifically, the study found:

[…] strong IQ gains of 0.5 IQ point per annum for East German conscripts in the 1990s, after the reunification of the country. An analysis of IQ, GDP, and educational gains in 16 German federal states between 1990 and 1998 shows that IQ gains had a .89 correlation with GDP gains and a .78 correlation with educational gains. The short time frame excludes significant effects of biological or genetic factors on IQ gains. These observations suggest a causal direction from GDP and education to IQ. (Roivainen, 2012)

Until last year, Germany had conscription for all male citizens, so this finding couldn’t result from biased sampling of the East German population. The samples were the population.

Wait a minute …

Or were they? Germans could dodge the draft on several grounds: conscientious objection; service in international aid organizations; illness; drug abuse; delinquency; criminality, etc. In total, over half of all German men evaded military service:

The post-Cold War downsizing of the Bundeswehr led to a considerable decrease in demand for young conscripts. Of all men reaching draftable age, less than one half actually served. In 2005 about 15% served in the military, while 31% performed civilian service or some other form of alternative service. More than 36% were screened out for medical reasons. This percentage was lower in the past (15% in 2003), but to avoid drafting more men than needed, medical standards had been raised. The remainder includes those who were exempt for various reasons, but is mostly made up of men who were not drafted because the military had already reached its recruitment goals. This had led to discussions about "draft equality" ("Wehrgerechtigkeit"), which is the principle that the draft should have applied equally and non-discriminatorily to all men. (Wikipedia, Conscription in Germany)

So the possibility of sampling bias is back on the table. Is there reason to believe that this sampling of the East German population changed between 1990 and 2006?

There were in fact two changes:


Increased sampling of the highly educated

The end of the Cold War brought a steady process of deindustrialization to the former East Germany. Full employment gave way to chronic double digit unemployment. As a result, highly educated Ossies increasingly saw military service as a possible career.

This change has been commented upon in a recent New York Times article:

In a recent study, two experts said that while only 16 percent of the German population of 82 million lives in the former East Germany, easterners make up 30 percent of military personnel. At the same time, said Michael Wolffsohn, a professor at the German Institute for International Security Affairs, and Maximilian Beenisch, a social scientist, easterners with higher educational qualifications were drawn to the military because of a lack of alternative opportunities in eastern Germany, where unemployment is higher than in the west. (New York Times, 2011)

The same point is made by Roivainen himself:

In a Bundeswehr survey from 1993, attitudes toward the army were more positive among East German youth and those planning a career in practical vocations than among West German youth and those wishing to study further. (Roivainen, 2012)

Decreased sampling of troubled youth

With reunification, a more liberal social environment in East Germany brought about an apparent rise in IQ scores. Fewer low IQ individuals were present in the sort of institutional settings where IQ tests were routinely administered.

Many of these individuals were juvenile delinquents. In Germany, delinquency leads to exclusion from military service: “Delinquents sentenced to more than a year or charged with a felony against peace, democracy, the state or state security were not drafted for military service” (Wikipedia, Conscription in Germany).

As I noted in an earlier post, truancy was less common in the old East Germany than in the old West Germany because punishment was harsher. This factor had the effect of depressing IQ scores in East Germany, since potentially truant individuals were more likely to be tested for IQ in East German classrooms than in West German ones.

The same reasoning applies to juvenile delinquency. With reunification, East German youth entered a less controlled social environment. Relatively low-skilled factory work also disappeared. The result was a sharp rise in youth violence and delinquency:

Most of the increase in German youth violence has been encountered in what was communist East Germany before the reunification. Youth violence in the east is 70 percent higher than in the west, a factor linked to the exposure of eastern youth to greater poverty and unemployment than their West German peers. (Siegel & Welsh, 2009, p. 577)

From 1990 delinquency began increasing in the region of East Germany and has reached critical proportions. There are three general types of delinquency: traditional delinquency, new kinds of delinquency from outside the region, and delinquency associated with the unification of Germany. Traditional delinquency has increased in nearly all significant categories of crime: theft, fraud, robbery, murder (homicides generally) and rape. The new category of crimes consists of drug trafficking and abuse, car theft, and environmental crimes. Violence against foreigners in the former East Germany is also significant. Crimes committed in association with unification include profiteering, fraud, and financial manipulations (Buchholz, 1993)

Conclusion

There is little doubt that the mean IQ of East German conscripts rose dramatically between 1990 and 2006. This increase, however, most likely reflected the changing composition of the pool of candidates for military service. Because of rising unemployment, high IQ individuals were much more likely to consider military service in 2006 than they were in 1900. Meanwhile, because of a more liberal social environment, low IQ individuals were much more prone to delinquency and being excluded from conscription.

But why do these gains in IQ correlate so highly, on a state-by-state basis, with gains in GDP and also, to a lesser extent, with gains in education? These high correlations are actually not so surprising given the tight relationship between the positive changes to East German society (rise in GDP due to expansion of the market economy) and the more negative ones (rise in unemployment due to deindustrialization and rise in juvenile delinquency due to social liberalization). These changes were two sides of the same coin.

By way of example, there existed in the 19th century a significant correlation between the salaries of Protestant ministers in Boston and the price of rum in Havana. Is this proof that something underhanded was going on? Hardly. The two trends were part of a larger worldwide cycle of economic upturns and downturns.

As Roivainen himself points out, the correlations with GDP and education are simply an artefact of the east-west divide within Germany:

However, within western Germany, the correlation between IQ gain and GDP gain was .03 and between IQ gain and educational gain −0.12. Thus, the strong correlations based on calculations involving all states mainly reflect the east–west divide and its gradual narrowing (Roivainen, 2012)

References

Buchholz, E. (1993). Social changes, crime and police in the former GDR (German Democratic Republic), from Social Change, Crime and Police: International Conference, June 1-4, 1992, Budapest, Hungary, pp. 115-121, (eds., Jozsef Vigh and Geza Katon). https://www.ncjrs.gov/App/publications/Abstract.aspx?id=144804

Cowell, A. (2011). The draft ends in Germany, but questions of identity endure, New York Times, June 30, http://www.nytimes.com/2011/07/01/world/europe/01germany.html?pagewanted=all

Leipzig, a demographic analysis. Germany, Saxony and Leipzig http://demographicleipzig.wordpress.com/2012/06/13/germanysaxonyleipzig/

Roivainen, E. (2012). Economic, educational, and IQ gains in eastern Germany 1990–2006, Intelligence, 40, 571–575. https://lesacreduprintemps19.files.wordpress.com/2012/09/roivainen.pdf

Siegel, L.J. & B. Welsh. (2009). Juvenile Delinquency: Theory, Practice, and Law, Cengage Learning.

Wikipedia, Conscription in Germany, http://en.wikipedia.org/wiki/Conscription_in_Germany

Growing your own merchant class

Sign for an inn, late 17th century, New France. Many merchants began as innkeepers who provided not only accommodation but also other services. (source)

I have just published an article in the journal Advances in Anthropology:  Tay-Sachs and French Canadians: A Case ofGene-Culture Co-Evolution? Comments are welcome.

Abstract

Tay-Sachs, an inherited neurological disorder, is unusually common among French Canadians from eastern Quebec. Two alleles are responsible, one being specific to the north shore of the St. Lawrence and the other to the south shore. This pattern of convergent evolution suggests the presence of a selection pressure limited to eastern Quebec. Both alleles probably arose after the British conquest of Quebec in 1759 or at least were uncommon previously. To explain the high incidence of Tay-Sachs among Ashkenazi Jews, some authors have invoked heterozygote advantage, i.e., heterozygous individuals enjoy a higher rate of neuronal growth, and thus greater learning capacity, without the neurological deterioration of homozygous individuals. Such an advantage would have helped Ashkenazim perform the mental effort required for work in trade and crafts. A similar situation may have developed in eastern Quebec, where the relative scarcity of British and American merchants made it easier for French Canadians to enter occupations that required literacy, numeracy, and future time orientation.

Reference

Frost, P. (2012). Tay-Sachs and French Canadians: A Case of Gene-Culture Co-Evolution? Advances in Anthropology, 2 (3), 132-138.

http://www.scirp.org/journal/PaperInformation.aspx?paperID=21701

Our brideprice culture

Dr. Griskevicius speaking on the mating game. The economic consequences are big. (source)

The mate market isn’t what it used to be. As late as the mid-1970s, single women outnumbered single men at all reproductive ages. A reversal then took place throughout the Western world. The “operational sex ratio” slipped from male scarcity to parity and then to a male surplus by the late 1980s. Why? The immediate reason was the large number of divorced male baby-boomers reentering the mate market … and competing for a smaller number of female baby-busters (Ni Bhrolchain & Sigle-Rushton, 2005; Pedersen, 1991).

Yet behind this immediate reason stood two longer-term causes. First, divorce had become much easier. Men were now freer to act on an unconscious command that all men are born with: Look for a younger wife if your current one no longer looks fertile … assuming of course you’ve managed to survive into your fifth decade of life.

This leads us to the second cause. By the late 20th century, male mortality had greatly declined. More men were now living throughout the entire reproductive age bracket. Keep in mind that around 105 boys are born for every 100 girls. In the past, this sex ratio balanced out within the first few years of life because of higher mortality among male infants. By the 1980s, it wasn’t balancing out until the mid-40s or even later.

Will the operational sex ratio return to normal in the future? No, but it may stop getting worse. The trend toward male longevity seems to have almost maxed out for all reproductive ages. And fewer older men will be reentering the mate market—the baby boomers are getting a bit old for that sort of thing. But then we’ll probably also see more polygyny of the non-serial kind. Simultaneous polygyny is hard to measure, since it’s illegal, but it seems to be a growing phenomenon. The incidence of gonorrhea and chlamydial infection is now higher in women than in men, an indication that the population of promiscuous individuals is disproportionately female (Miller et al., 2004).

Even if we ignore unofficial polygyny, the “official” ratio of single men to single women shows no signs of declining, even at older ages (Soma, 2008). In Germany, single men now outnumber single women up to the age of 60 (Glowsky, 2007). To make a long story short, young nubile women are a limited resource. The more you deregulate the marriage market and the more you reduce male mortality, the fewer of them there’ll be to go around.

And the consequences?

As men become increasingly outnumbered on the mate market, what will be the consequences? Until recently, research has focused on the social effects. Permanently single men have less of a stake in maintaining the status quo and tend to be drawn to revolutionary movements of one sort or another (Hudson & Den Boer, 2002).

Now it appears that there are also economic consequences:

Findings show that male-biased sex ratios (an abundance of men) lead men to discount the future and desire immediate rewards. Male-biased sex ratios decreased men’s desire to save for the future and increased their willingness to incur debt for immediate expenditures. Sex ratio appears to influence behavior by increasing the intensity of same-sex competition for mates. Accordingly, a scarcity of women led people to expect men to spend more money during courtship, such as by paying more for engagement rings. (Griskevicius et al, 2012)

In the above study, the operational sex ratio (ratio of adult unmarried men to adult unmarried women) was compared to the number of credit cards owned and the amount of consumer debt for 134 American cities. Both indices significantly correlated with the operational sex ratio. By way of example, the authors point to two cities in Georgia:

Macon, Georgia, and Columbus, Georgia, are two cities in the southeastern United States that are less than a hundred miles apart. Both cities share a similar historical heritage and economic climate. Despite these similarities, the residents of each city have drastically different spending habits: The average consumer debt of people living in Columbus is an astounding 2.7 standard deviations higher than that of people living in Macon—a difference of $3,479 per consumer […]

 

We suggest that this difference in debt might be linked to an often overlooked difference between the two cities: the ratio of single adult men to women in each area. Whereas in Macon there are only 0.78 single men for every woman, in Columbus there are 1.18 single men for every woman. (Griskevicius et al, 2012)

The United States, like most of the Western world, is moving from a dowry culture to a brideprice culture (Sailer, 2008). Single men now have to pay to get a wife, and the price is getting steeper.

But this change isn’t just a matter of “Who pays the money?” It’s also “How is it spent?” A dowry is an investment in the future. It often takes the form of a down-payment by the bride’s family on the newlyweds’ home. In contrast, a bride price usually goes toward immediate consumption. A single man has to spend more on dating, which itself is becoming a longer, multiyear process. He’ll be expected to pay off his bride’s credit card debt. And he’ll have to spend more on the wedding.

The difference between dowry and bride price reflects a difference in mate-choice criteria. Whereas men judge women on the basis of youth and perceived fertility, women judge men on the basis of perceived social and economic status. A shift towards bride price thus compels men to boost their image by spending more now and leaving less for the future.

There is a widespread belief that consumers will save and pay off their debts if offered the right economic incentives, i.e., higher interest rates, tax deductions, etc. This belief ignores the changes that have occurred to the dynamics of mating over the last few decades. Men will keep on going heavily into debt because that’s the price of getting a mate. Women will continue to borrow beyond their means, knowing that they can convert their debt burden into bride price. 

References

Glowsky, D. (2007). Why do German men marry women from less developed countries? SOEP papers on Multidisciplinary Panel Data Research #61
http://www.dix.de/soeppapers

Griskevicius, V., J.M. Tybur, J.M. Ackerman, A.W. Delton, T.E. Robertson, & A.E. White. (2012). The financial consequences of too many men: Sex ratio effects on saving, borrowing, and spending, Journal of Personality and Social Psychology, 102(1), 69-80.

Griskevicius, V. (2012). Youtube video http://www.youtube.com/watch?v=PY706EGtUnY

Hudson, V.M., & A. Den Boer. (2002). A surplus of men, a deficit of peace. Security and sex ratios in Asia’s largest states, International Security, 26, 5-38.

Miller, W.C., C.A. Ford, M. Morris, M.S. Handcock, J.L. Schmitz, M.M. Hobbs, M.S. Cohen, K.M. Harris, & J.R. Udry. (2009). Prevalence of chlamydial and gonococcal infections among young adults in the United States, Journal of the American Medical Association, 291, 2229-2236.

Ni Bhrolchain, M. & W. Sigle-Rushton. (2005). Partner supply in Britain and the U.S. Estimates and gender contrasts, Population, 60, 37-64.

Pedersen, F.A. (1991). Secular trends in human sex ratios: Their influence on individual and family behavior, Human Nature, 2, 271-291.

Sailer, S. (2008). Bachelorettes in debt – the new reverse dowry system, Steve Sailer’s iSteve blog, November 7  http://isteve.blogspot.ca/2008/11/bachelorettes-in-debt-new-reverse-dowry.html

Soma. (2008). The new interactive singles map http://www.xoxosoma.com/singles/