Gene and Behavior: A Comlex Relationship

Sometimes lessons in biology some unexpectedly, as when a rottweiler appeared over my right shoulder while I was kneeling to trim some rose bushes early this summer. He was large--more so for being at eye level--and he was wearing a collar made of chain links large enough to anchor a small ship. I turned to face him as an archive of rottweiler news stories came up from long-term memory, none of them happy, all of them populated by mutilated people and by dogs destroyed at the direction of the authorities.

What to do next? How did I appear to this animal? Did I look like a threat? Did I look like lunch? He certainly wasn't behaving in a menacing manner, but maybe he was waiting for the right moment to express his breed's well-known, vicious disposition. One of us had to do something, I figured, although reason, the pride of Homo sapiens, was little comfort in the face of the evolutionary legacy of Canis familiaris: powerful jaws and teeth adapted to gripping and tearing. I extended my hand slowly, and--he licked it. The dog worked his way up to my face with a tongue as broad and soggy as a kitchen sponge, and, as his owner appeared, I expressed my relief at seeing such friendly behavior from a representative of so notorious a breed. The young man replied, "These dogs get a bad rap. People say they're born mean. Look at Caesar, here. Ain't no dog born mean. You got to teach them to be mean."

In their different ways, Caesar and his human friend raised long-standing questions about the roots of animal behavior, including behavior in our own species. Are behaviors inbred, written indelibly in our genes as immutable biological imperatives, or is the environment more important in shaping our thoughts and actions? Such questions cycle through society repeatedly, forming the public nexus of the "nature vs. nurture controversy," a strange locution to biologists, who recognize that behaviors exist only in the context of environmental influence. Nonetheless, the debate flares anew every few years, reigniting in response to genetic analyses of traits such as intelligence, criminality, or homosexuality, characteristics freighted with social, political, and legal meaning.

Sir Francis Galton (1822-1911) was the first scientist to study heredity and human behavior systematically. He focused on behavioral correlations within families and developed a few research techniques still in use today--twin studies, for example. Galton also arrived at some interesting conclusions, including this 1907 summary of the inheritance of criminal tendencies: The ideal criminal has marked peculiarities of character: his conscience is almost deficient, his instincts are vicious, his power of self-control is very weak, and he usually detests continuous labor. The absence of self-control is due to ungovernable temper, to passion, or to mere imbecility, and the conditions that determine the particular descriptions of crime are the character of the instincts and of the temptation.

The perpetuation of the criminal class by heredity is a question difficult to grapple with on many accounts.... It is, however, easy to show that the criminal nature tends to be inherited.... 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. [Galton, Inquiry into Human Faculty and its Development, 2nd edition (London: J.M. Dent & Sons, Ltd., 1907).]

This passage demonstrates Galton's conviction about the hereditary basis of criminal behavior, by no means established even now, but he wrote elsewhere in the same volume about the "difficulty of distinguishing that part of (man's) character which has been acquired through education and circumstance, and that which was in the original grain of his constitution." 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.

Behavioral genetics

The term "genetics" did not even appear until 1909, only two years before Galton's death, but with or without a formal name, the study of heredity always has been, at its core, the study of biological variation. 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. Is intelligence the ability to solve a certain type of problem? The ability to make one's way successfully in the world? The ability to score well on an IQ test? 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. Because the experimental animals performed better than controls on a series of traditional tests of learning, the press dubbed this gene "the smart gene" and the "I.Q. gene," as if improved memory were the central, or even sole, criterion for defining intelligence. 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. This is a bit like using batting average to define hitting prowess in baseball. A high average may indicate ability, but it does not define the essence of the trait.

* Behaviors, like all complex traits, involve multiple genes, a reality that complicates the search for genetic contributions.

* As with much other research in genetics, studies of genes and behavior require analysis of families and populations for comparison of those who have the trait in question with those who do not. The result often is a statement of "heritability," a statistical construct that estimates the amount of variation in a population that is attributable to genetic factors. The explanatory