Why are human beings so different from one another? Why are some people tall, some short; some brown-eyed, some blue-eyed? The fact that daughters and sons tend to look like mothers and fathers suggests that physical features are heritable, and therefore due in part to genes. But what about behavior? Why, in the very same family, are some children assertive, others shy? Why are some highly emotional, others more reserved and "Logical?" Are these traits heritable too? Co differences in these traits among individuals also have a genetic basis?

The idea that at least some of the variability we see in human behavior and personality is heritable, and therefore genetically determined, would certainly come as no surprise to most animal breeders. For at least half a millennium or more, animals have been bred specifically to reinforce certain behavioral or personality traits. Some dogs, ranging in size from tiny terriers to massive pit bulls or Dobermans, have been bred for their aggressive nature. Others, such as collies or spaniels, faithfully transmit a docile, loving nature from generation to generation. Still others have been bred to carry out specific tasks related to hunting, or managing flocks of animals. In the laboratory, rats and mice have been selectively bred for many generations to create strains that are fearful or aggressive. These strains pass on their personality differences each time they breed. No one seriously questions the role of genes in the development of animal behavior, or of inheritance in passing these traits from one generation to the next. Yet we are reluctant to acknowledge a similar role of genes in guiding human behavior.

At a deeper level, we know that the lives of cells are closely governed by genes, whether those cells are individual, free-living organisms such as yeast and amoebas, or the interactive cells that make up our own bodies. Single-cell organisms show definite signs of behavior. Within ourselves, the cells charged with managing how we react, how we behave, are all located within our nervous systems. And it is precisely when we come to the cellularly more complex nervous system that the issue of genes and behavior becomes more complicated. The major complication arises becase, more than in any other organ or system in the body, the behavior of cells in the nervous system is affected not only by genes, but by the external environment. Nerve cells are our window onto the world around us. We use the various images impressed onto our nerve cells to formulate responses to our environment, and this experience of our environment - and our responses to it - are remembered. Nerve cells are altered by contact with the environment, in ways that are still only crudely understood, but which alter the way we respond to the same information when it appears in the environment again.

The role of genes in governing behavior remains one of the most controversial topics in all of human biology. Early in this century, over-eager promotion of a genetic basis for behavior led to the initial silly excesses, but ultimately to the stunning horrors, of eugenics. Subsequent reactions to these excesses, within both the scientific community and society as a whole, led to a nearly complete dismissal of a role for genes in human behavior for many decades. We are slowly coming back to a more balanced view. A detailed study of the biological basis of behavior in animals, from the simplest single-cell creatures through the most complex mammals, shows that genes play a very important role in guiding behavior. Inheritance studies in humans, especially those involving twins reared together or apart, indicate clearly that humans are no exception. The variability we see around us today in the way humans respond in a given situation is to a large extent influenced by the variability in their genetic makeup.

Part of our concern about the role of genes in determining human behavior surely lies in our concern about free will and personal responsibility. All legal and moral systems assume that individuals are free to choose among alternative courses of behavior; individual responsibility has no meaning in the absence of unimpeded choice. But if our every behavior can be predicted from what is written into our genes before we are born, what does that say about our freedom to choose, or about responsibility for the individual choices we make? The opposite view, that we come into the world as some sort of blank template and that what we become, as cognizant adults, is simply the totality of our previous experiences, renders our behavior no less predictable - and our freedom to choose and act no less constrained - than if we were simply the sum of our genes.

As we begin the new millennium, we will witness the completion of one of the boldest undertakings in the history of biology - the Human Genome Project. This project, when completed in 2003, will provide us with a complete catalog of every gene involved in the construction and operation of an entire human being. It will take another decade or two at least to sort out what most of these genes do, but beyond question many of them will be involved in determining human behavior. Understanding the extent to which certain behaviors are genetically determined will provide us with better insight into human nature and give us a more realistic sense of the extent to which behavior can be modified by external interventions. That in itself will be of enormous value. But we will also have at our command the technology to alter those genes. What will we do with this new information? How will we explore these new possibilities? Will it be possible to use this information for the betterment of humankind, or will we simply plunge headlong into a renewed and even more disturbing flirtation with eugenics?