Slow-fast analysis of a multi-group asset flow model with implications for the dynamics of wealth.

The multi-group asset flow model is a nonlinear dynamical system originally developed as a tool for understanding the behavioral foundations of market phenomena such as flash crashes and price bubbles. In this paper we use a modification of this model to analyze the dynamics of a single-asset market in situations when the trading rates of investors (i.e., their desire to exchange stock for cash) are prescribed ahead of time and independent of the state of the market. Under the assumption of fast trading compared to the time-rate of change in the prescribed trading rates we decompose the dynamics of the system to fast and slow components. We use the model to derive a variety of observations regarding the dynamics of price and investors' wealth, and the dependence of these quantities on the prescribed trading rates. In particular, we show that strategies with constant trading rates, which represent the well-known constant-rebalanced portfolio (CRP) strategies, are optimal in the sense that they minimize investment risks. In contrast, we show that investors pursuing non-CRP strategies are at risk of loss of wealth, as a result of the slow system not being integrable in the sense that cyclic trading rates do not always result in periodic price variations.

Smiling in photographs: childhood similarities between sexes become differences constant in adulthood.

Contemporary published photographs of 1639 children, 200 older teenagers, and 304 adults in North America were analyzed for the smiling pattern (full, partial, or none) exhibited by individuals by sex. For each of the three age groups and for both sexes, most individuals posed with a full smile. No statistically significant differences in smiling pattern were present between the sexes for photographs of children at preschool and grade-school age. By teenage years, and even more for the adults, there were statistically significant differences between the sexes with regard to a smiling facial expression. In those cases, more female than male subjects smiled fully, whereas more male than female subjects did not smile. Examination of photographs of well-known persons longitudinally through adulthood showed that individuals tended to be consistent in smiling pattern. There was no significant sex difference for this relative constancy of facial expression in posed photographs.

Efficient validation of teaching and learning using multiple-choice exams.

One purpose of this study was to quantify, by means of single-format, multiple-choice questions at the beginning and end of the course, the extent to which first-year medical students learn neuroscience material from an introductory course in their curriculum. Compared with their precourse test performance (mean = 41.8%), collectively, the students nearly doubled their grade by the end of the course (mean = 81.4%). Their scores in subcategories of the material improved in inverse proportion to what they knew initially. A second goal was to evaluate a two-dimensional, computer-generated matrix as a way to assess test question validity and value. The evaluation of individual test questions as assessed from the matrix often, but not always, was similar to the classical pedagogical analysis that uses difficulty and discrimination indexes. Strengths of the matrix are its ability to render data as a gestalt, as well as flexibility and intuitive ease of use.