Do humans choose partners that physically resemble themselves

In this thesis, seven papers concerning life history, mate choice and parental investment strategies are presented. Data was compiled from old church books from two parishes in Central Norway from 1700-1900, Soknedal and Smola (Paper I-IV). Also the results of three questionnaires are presented (Paper V-VII). Results show that access to stable resources was the main predictor of number of children born and number of grandchildren produced within and between two human populations (Paper I). The two studied villages (Soknedal and Smola) were at the same latitude, but had different resource foundation in that the coastal parish (Smola) had access to year round fishing. This extra resource affected most life history traits of particularly low class women, who produced significantly more grandchildren than their sisters without this stable resource. Birth rank and family size also affected the life history of children (Paper II). Here we considered the survival rate, the probabilities of becoming married and migration rate from the home parish in relation to status of mother, family size, sex of the children, birth rank, sex ratio of siblings, year of birth and age of the mother. We found that the future reproductive value of boys was greatly affected by both family size and birth rank, in that boys from larger families and boys late in birth rank had a lower probability of getting married and a higher probability of migrating from the parish. No such relations were found for girls. We conclude that a difference in the access to parental resources during childhood affected the life history of boys, but not that of girls. There was no support for longer birth intervals between the births of two boys compared to other sex combinations of children (Paper III). Short birth intervals (less than two years) between two children led to higher mortality among both the first-born and second-born in such combinations There were significant differences in birth interval between high and low status women in both parishes. Also the poorer parish of Soknedal had longer birth intervals than Smola for both status groups. The survival rate increased significantly when birth intervals exceeded 2 years, both for a child and its next sibling. The optimal birth rate in Soknedal seems to be slightly over three years. In a multilinear regression analysis, the number of children of mother was, not surprisingly, the most important variable in explaining the variation in birth interval. Laterborn children had longer birth intervals (Paper III). In Paper IV we tested the inbreeding avoidance between related individuals in Soknedal parish, and found this to be significant. We found, however, no statistically significant differences in fertility between the three groups called non-locally, consanguineous and locally married couples. Physical variables in human mate choice were tested on a sample of students (Paper V). There was a significant correlation between the age of a man and a woman and the height of the two individuals in a pair. However, none of the correlations or cross-correlations between height, weight, hair colour or eye colour were statistically significant. Within a pair there was a highly significant positive correlation between the attractiveness of a man and a woman. We also tested if strangers could pick out mates by facial looks. 101 test persons were presented a series of four photos, two males and two females and asked to pick out the pair. The pair was correctly picked out in about 40 % of the cases, which was higher than random. Altogether almost 90 percent of the test persons were able to pick out the pair more frequently than random. There was no significant difference between the sexes in their ability to pick out the right pair from photos. Paper VI presents results concerning female view of the male commitment into a relationship, his economic status variables and his ambition levels. We tested predictions derived from evolutionary biology concerning female mate choice through a questionnaire presented to female subscribers of a Norwegian magazine. The expectations of a stable economic wealth prior to engagement affected positively the length of relationships. The partner's economic stability, the respondent's perceived economic satisfaction and the partner’s investment in children from previous relationships were significantly higher in an ongoing than in a broken relationship. These factors are predicted to be crucial to mate choice and ultimately fitness-enhancing. Finally, in Paper VII phenotypic levels of daily-life parental-offspring conflicts in two different social contexts were studied; 1) between biological parents and their offspring at home, and 2) between the adults and the same children in the nursery school. Parents and nursery school teachers were asked how the children acted in conflicting situations that frequently occur during a day. Parents reported a higher level of conflict with their children than did teachers from nursery schools. Parents did not experience differences in conflict level between boys and girls, while nursery school teachers did experience such differences. The results give support for the hypothesis that children have an innate and selected mechanism that guides them in different social contexts, and that they easily assess differences important for the level of care and attention they can expect to get. Children expect a higher investment from their parents than from their teachers, and therefore solicit more in the parent-offspring context. Such context-related behaviour among children will optimise their own social status and benefits in the form of resources and attention, and is the conceptual background for parent-offspring conflict.