In two-child families containing at least one boy, the expected probability that such a family has two boys is 1/3, provided that the boy/girl (B/G) ratio is 1.0 and the population to which they belong has a binomial distribution of BB, (BG GB), and GG families. It is commonly known that in most human populations the sex ratio at birth (i.e., the ratio of the number of boys to the number of girls) is greater than 1.0. Teachers and textbook writers seldom discuss the more realistic expected distributions in populations where the sex ratio is greater than 1.0. We present data from two federal surveys with sex ratios greater than 1.0 and find that the observed proportions of two boys in families of size 2 with at least one boy range from 0.3335 to 0.3941. It has been reported in the literature that the probability (p) of a male birth is subject to both within-sibship variation (Poisson variation), for which our data are suggestive, and possibly also between-sibship variation (Lexis variation). These deviations (biases) from the assumptions of a simple binomial distribution are involved in the calculation of values of p and standard 95% confidence intervals, thereby foiling attempts to make reliable statistical inferences from the data. Analysis of the data is also complicated by family planning that falsifies the assumption of randomness in the binomial gender distribution model. Families of size 2 (and their sex composition) are often discussed in a wider context. Overpopulation in some parts of the world has caused mass starvation and threatens to do the same worldwide unless the birth rate drops to agriculturally sustainable levels. Even if every woman of fertile age has only two children on average from now on, the world's population is predicted to continue growing toward 9 billion people by 2050. Other sociological problems are bound to follow. Although the birth rate in China has recently dropped, the average age of the population has risen, so that by 2035 it is projected that for each person over age 65 there will be just three working-age people. Furthermore, China's one-child policy has already led to a sex imbalance where there is a large excess of men for whom marriage and parentage is denied.

Stansfield and Carlton [Human Biology 79:255–260 (2007)] reported that the distributions of the combinations of the sexes in human sibships are binomial. They inferred that the probabilities of male births are equal and independent within and across all sibships. Here I argue that their argument is both invalid and false. Contrary to their inference, a binomial distribution may result when equal and counterbalancing measures of Poisson and Lexis variation are simultaneously present. These conditions are approximately met with respect to human births.

Natural selection, drift, and gene flow are the three major evolutionary forces at the origin of genetic diversity among human populations. To further explore these mechanisms, we present an innovative approach using various medical genetic markers and focusing on the Basque population. From this study we can confirm the important role of drift in this endogamous human group and can report some disorders related to founder effects. Most important, the peculiar distribution of various polymorphisms, such as blood group O, factor V Leiden, DF508, C282Y, and CCR5 D32 mutations, which are implicated in resistance to infection, hemostasis, or iron conservation, could be interpreted as an adaptive profile. Multidisciplinary data have shown that the Neolithic period arrived significantly later in this southwestern corner of Europe. We hypothesize that the long-lasting Paleolithic mode of life, especially regarding nutrition and microbial exposure, was at the origin of this selective pressure within this population of ancient local ancestry. This approach could open new avenues in the field of population genetics.

Genetic studies of the distribution of mitochondrial DNA (mtDNA) haplogroups in human populations residing within the Carpathian Mountain range have been scarce. We present an analysis of mtDNA haplogroup composition of the Boykos, Hutsuls, and Lemkos, three population groups of the Carpathian highlands. In our study Hutsuls had the highest frequency of subhaplogroup H1 in central and eastern Europe. Lemkos shared the highest frequency of haplogroup I ever reported and the highest frequency of haplogroup M^* in the region. MtDNA haplogroup frequencies in Boykos were different from most modern European populations. We interpreted these unique mtDNA frequencies to be evidence of diverse and dynamic population histories in the Carpathian highland region.

We report the incidence and distribution of Native American mtDNA haplogroups in nine villages across the Sierras Centrales archeological area, located in central Argentina. The aims of the study were (1) to investigate the relative incidence of native maternal lineages, (2) to determine whether or not the homogeneous pattern observed in a previous study persists at this larger scale, and (3) to ascertain the genetic affinities between the studied population and other native populations of the Southern Cone of South America. Of the 310 individuals from whom DNA was extracted, 249 (80.3%) were assigned to one of the founding native American haplogroups. This finding confirms the persistence at high prevalence of native maternal lineages in the rural populations of central Argentina. The haplogroup distribution is homogeneous in the population samples from Córdoba province, with haplogroups C and D always found at the highest frequencies. The sample from San Luis province, Tilisarao, presents a different genetic pattern, with haplogroups A and B being the most frequent. Principal components analysis and SAMOVA at the regional level show that the Córdoba, Patagonia, and Tierra del Fuego populations cluster together, which suggests a common origin.

Eleven short tandem repeat loci (CSF1PO, D3S1358, D5S818, D7S820, D8S1179, D13S317, D16S539, D18S51, D21S11, TH01, and TPOX) were investigated in 232 individuals from 6 Amazonian native tribes (Tiriyó, Waiãpi, Zoé, Urubu-Kaapor, Awa-Guajá, and Parakanã). We added the new data to a database that included five previously typed native populations from the same area (Wai Wai, Gavião, Zoró, Suruí, and Xavante). The results presented here concern this new data set, which accounts for 526 individuals in total. We tested whether major geographic or linguistic barriers to gene flow exist among such human groups and tried to find a possible anthropological or ethnological explanation for such patterns. We measured the average heterozygosity (H) and the number of alleles (N_A) and found that both are lower than values observed in populations of different ethnic backgrounds, such as European or African descendants. Despite such a result, we found high between-population variation; lower H and/or N_A values were obtained from four isolated tribes that came into contact with external nonnative populations in recent times (1921–1989). By applying analysis of molecular variance, generalized hierarchical modeling, and the Structure Bayesian analysis, we were not able to detect any significant geographic or linguistic barrier to gene flow. Geographic autocorrelation analysis suggests that the genetic structure of native Amazonian tribes is better explained by isolation by distance because the level of genetic similarity decreases according to linear geographic distance, reaching null or negative values at a scale of 300 km.

Choosing a spouse has always been an event subjected to strong social control. Consideration of socioprofessional elements is therefore necessary for an in-depth understanding of the microevolutionary consequences of marriage. Thus in this paper I explore the extent and causes of reproduction isolation between different socioprofessional groups of a 19th-century Italian community, Casalguidi. Various features of mate choice pattern were investigated by means of individual-level data. The results show that Casalguidi was a community with strong socioeconomic internal boundaries, in which social belonging mattered much more than geographic provenience. The bourgeoisie showed such a high degree of social homogamy that it was almost reproductively isolated. Much more openness and less isolation was found between the two categories of farm laborers and sharecroppers, and day laborers.