Sometimes a collaboration between individuals occupied in separate biological fields and the application of understandings from one field to the other leads to felicitous insights and new perspectives.

Our own experience has encompassed studies of the evolutionary bases of human behavior on the one hand and two decades of clinical experience with addicts of various kinds (and their families) on the other.

Departing from the conventional view that addiction arises solely from the life history of an individual or out of an obscure chemical imbalance, we have come to a formulation of the problem, rather, as one of the effects of group mechanisms upon the individual.

The dynamics residing within the entity we call a society affect all its members. There are those who can adapt themselves to group requirements and others who in some or many ways cannot.

This applies to all social groups of all creatures, whether animal or human. Very frequently manifestations that appear to us to be peculiarly human, when compared with the patterns of life of other animals, come to be thought of as due to our cultural endowment or to our specific civilization and as phenomena that therefore define a separation between our species and all others.

Language, love, politics, and the care of the sick are among many human propensities and predilections that come into this category. Yet everything human has its origin in an animal past, and such a view tends to prevent certain aspects of human behavior from being seen in a context of overall natural patterns, hindering full understanding of their significance.

The problem of addiction is certainly a human one, and it has not been thought of in terms of comparative behavior. The reason is simple.

Addiction does not occur in a natural state. Laboratory animals may be induced artificially to become addicted to most of the substances on which a human being may become physiologically or psychologically dependent, but this does not happen in feral conditions.

Nor, on the other hand, is the presence of ??omind??? in humans an explanation for the different behavior, since animals with no advanced neocortical development can become addicted in laboratory conditions.

Beyond the failure to view addiction in terms of overall natural processes-- or perhaps a part of that failure--is the tendency we have had to ask questions about the ??owhys??? of addiction in terms only of an addicted individual??Ts life.

We ask what personal problems led him to turn to drugs or alcohol for relief. Even if we take a step further and examine the social background of the addict, seeking a cause for his problems on a wider basis, this larger dimension is considered relevant only in terms of its effect on the individual; and so the answers we find, like the questions we pose, remain individual oriented. Since the study of the individual is the domain of the psychiatrist, the problems of addiction have come to be accepted as within his province.

A further question that must arise, of course, is how it can happen that addiction can arise biologically. This question has been asked by some, and answers to it have been sought in the physiology of the nervous system.

But this step again focuses on the individual, even when investigations are pursued into his genetic background; and so, while the question is right, the approach to answering it is limiting, since it leads no further than the previous ones--to the individual. Yet it is indeed in neurophysiology that we may begin to find clues to the larger pattern.

The nervous system is more than a recipient of stimuli and regulator of an organism??Ts behavior. It is a repository of reflex responses that connect the individual to his phylogenetic past and is also a regulator of interactions between the individual and the present society of which he is a part.

What we call social pressures are conveyed to an individual, and he reacts to them, not only through his understanding but also through direct neural responses, so that in this sense the nervous system is the mediator between an individual and a society in a way analogous to the role of the hormones in mediating between the behavior of cells and the needs of the whole organism.

In binding individuals to the needs of their societies, their nervous systems serve to integrate group well-being. To see this clearly, it is helpful to look at some group mechanisms in the breeding groups of other species, and we may then see how these throw light on otherwise puzzling human behavior.

The pioneer experiments of R.N. Chapman (1928) showed that, in an enclosed environment in which the nutrient medium was a layer of flour two centimeters deep, a steady ceiling population of the flour beetle (Tribolium confusum) would ultimately be obtained.

An experimentally repeatable, almost constant density of individuals per gram of flour was finally arrived at, whether the culture was started with one pair of adults or many pairs or whether the volume of flour was small or large.

Of many subsequent workers, D.S. MacLagan (1962) performed parallel experiments with Tribolium and Sitophilus, the grain weevil. He found that there was a drop in the number of eggs laid per female associated with crowding, and he concluded that natural populations as well as experimental ones ??oautomatically check their own increase by virtue of this density effect, and that the organism itself imposes the ultimate limit to its own abundance when all other factors (biotic and  physical) have failed??? (p. 452).

Both Tribolium confusum and its close relative T. castaneum, when adult, have glands in the thorax and abdomen that produce an irritant gas that P. Alexander and D. H.R. Barton (1943) identified as ethylquinone. The glands are stimulated to liberate this gas by disturbance and crowding.

In crystalline form, it is lethal to first-instar larvae; as a gas it induces developmental abnormalities in late larvae and pupae, and it probably has a depressing effect on the well-being of the adults (Roth and Howland 1958).

The now classic experiments of C.M. Breder and C.W. Coates (1932) showed that, in tanks containing an equal volume of water, whether a single gravid female or a number of guppies (Labistes reticulatus) were placed in them, it took only about 20 weeks for the same constant population of nine or ten fish to be reached in each.

Surplus individuals were cannibalized. Tadpoles overcrowded in their tanks excrete into the water metabolites that have the effect of stunting growth until the smaller individuals die off and the population is adjusted to an uncrowded condition (Richard 1958).

Socially induced mortality occurs also in birds, and among them social status becomes a factor. The most subordinate members of a population may be inhibited from breeding at all.

Those a little higher in rank may achieve a nest and mate and perhaps even eggs, but when environmental conditions impose a necessity for a reduction in the number of young birds being reared, the stress falls more tellingly on them than on better-established and higher-ranking members of the community.

A wide variety of species-specific mechanisms are brought into play, from reduced egg production or the destruction of eggs to the killing of young, but for our present purpose it is sufficient to note that the reduction of the threshold of resistance to parasitic infestation is one of the many manifestations of crowding stress that have a homeostatic effect.

D. Lack (1954, chapter 54), in an extensive review of mortality attributable to disease in birds, noted that, when they are in good condition, such birds as the red grouse (Lagopus lagopus) can carry a considerable burden of internal parasites without injury but that, if the quality of their staple food plant is affected by harsh weather or by unusually extensive damage by the heather beetle, then the birds??T threshold of resistance is lowered so that the lower ranking appear to die of parasitic disease.

The element of status in the survival of birds in crowded conditions was noted in an extreme manifestation by A.A. Allen (1934). He observed in a captive group of ruffed grouse (Bonasa umbellus L.) what he called an intimidation display. ??oA bird that has been completely subjugated . . . is subject to attack from every other bird in the enclosure. He has developed an inferiorism and usually, unless removed, he remains in a corner until he dies, not from mechanical injury nor from starvation, but from some sort of nervous shock, and death is likely to occur within 24 hours.???

V.C. Wynne-Edwards (1962) has commented that the function of hierarchy is to identify surplus individuals whenever environmental necessities require a reduction of population. Wild mammals respond no less than other creatures to population density. In North Wales Brambell and his associates made the discovery in the rabbit (Oryctolagus cuniculus) that an average of 64 percent of embryos  conceived perish before birth, usually by the twelfth day of pregnancy.

The arrested embryos are not aborted, but their tissue is broken down and resorbed by the uterus, leaving nothing but an impermanent scar (Thompson and Worden 1956, pp. 112-113). The percentage of embryos resorbed is responsive to environmental conditions.

These are but arbitrary examples (that could be multiplied almost endlessly) taken from insect, fish, amphibian, avian, and mammalian societies, to give some idea of the types of social mechanisms to which we are referring.

V.C. Wynne-Edwards states that there can remain no doubt that populations are effectively self-limiting, ??oand the inference must be very strong that selection has perfected the adaptations so that population densities always tend to balance themselves at the optimum level??? (1962, p. 498).

In his encyclopedic work, he has shown that there is probably no species that does not have some built-in method of population control that effectively regulates the density of its breeding groups or societies. Indeed it is clear that this must be so, since any population that failed to effect this regulation would very soon strip its habitat of the resources necessary for its sustenance and thus promote its own extinction.

For each species, the range of ??opersonal space??? required by individuals varies, but the work of such researchers as J.B. Calhoun (1962, 1963) has made us aware of the gross distortions of normal behavior that occur when this space requirement is infringed. The more enlightened curators of zoos have recently come to recognize the modifying effects on the natural behavior of animals of cages that confine their living space too closely, and we ourselves have become aware of our own need for ??opersonal space??? and of human responses ranging from mild irritation all the way to violent aggression that may occur when it is invaded.

The potential for these responses is carried genetically in all species, and H. Selye (1950) has shown that social stress can have depressing and injurious effects on the animal body just as severe as those produced by disease, hunger, or fatigue.

But the mediating agency between the environment and the individual is the nervous system, in that the nervous system not only brings awareness of population pressures to the individual but also sets in motion the adaptive responses, whether physiological or behavioral.