The true state of the case appears to be that the criminal population receives steady accessions from those who, without having strongly marked criminal natures, do nevertheless belong to a type of humanity that is exceedingly ill suited to play a respectable part in our modern civilization, though it is well suited to flourish under half-savage conditions, being naturally both healthy and prolific. The difficulty persists, notwithstanding an explosion of data about human genes and the development of molecular and statistical tools that Galton could not have imagined. Human behavioral genetics, a relatively new field, seeks to understand both the genetic and environmental contributions to individual variations in human behavior. That is not an easy task, for the following reasons. It often is difficult to define the behavior in question.
Intelligence is a classic example.
During the late summer of 1999, a Princeton molecular biologist published the results of impressive research in which he enhanced the ability to learn in mice by inserting a gene that codes for a protein in brain cells known to be associated with memory. In reality, there is no universal agreement on the definition of intelligence, even among those who study it for a living. Having established a definition for research purposes, the investigator still must measure the behavior with acceptable degrees of validity and reliability. That is especially difficult for basic personality traits, such as shyness or assertiveness, which are the subject of much current research. Sometimes there is an interesting conflation of definition and measurement, as in the case of IQ tests, where the test score itself has come to define the trait it measures.