Typically the increasing lawlessness of young children in schools is blamed on “difficult home circumstances”, inadequate parenting, or even a general breakdown in society. But a new review of 30 years of scientific research has revealed a startling new dimension: classroom crowding.

Numerous scientifically controlled studies conducted by psychologists in the UK and US, and brought together for the first time in this new review, show that increasingly crowded nursery and primary schools lead to children becoming more hostile and openly aggressive: shouting, pushing, kicking, punching and biting. In the light of the Government’s recent scrapping of statutory guidelines on pupil-space ratios, and the likelihood that class size will be an election issue, the implications of these findings are enormous.

Most of the research has been done with children directly: in the play-ground, in classrooms, in extra-curricular settings in schools. But the first study to focus scientists’ minds on the effects of crowding featured, perhaps inevitably, laboratory rats. Published in the early Sixties, this was a study of animal crowding conducted on rats which had otherwise been kept under very favourable conditions. Food and water was available whenever they wished, and plenty of nesting materials were provided. In fact, everything was very comfortable for them, except for one thing. Space.

As the rat population increased, the amount of space available remained constant. After a time, when things became quite crowded, the behaviour of the rats began to change. Some of the young male rats became extremely aggressive. Others withdrew from social contact totally. It became what the researchers called a “behavioural sink”.

Of course, extrapolations from rats to human culture are highly speculative, and our experiences of our environment are mediated by the tremendous adaptability of the human being. Nevertheless, subsequent research has shown that the study on rats bears uncomfortably close comparison to what is happening to children in schools.

The first systematic study on the effects of crowding on children was carried out at Oxford University 30 years ago. The researcher, Corrine Hutt, studied children aged three to eight who were playing indoors in groups of differing sizes. As the groups were increased from small (six children) to medium (between seven and 11 children) and large (more than 12), the incidence of aggressive behaviour increased also, both between the children and towards furniture and toys in the room.

At its height, children were spending 16 per cent of their play time in aggressive or destructive behaviour. Hutt felt that the range of responses of the children paralleled the behaviour of rats under high-density conditions.

Intrigued by these findings, I carried out a larger and more systematic study on children in a private nursery school. In this particular playgroup a maximum of 30 children played together daily. Over three months, I systematically observed 20 of them, one by one, during the twice-daily free-play sessions. The number of children in the play session on any one day varied naturally.

For instance, some children attended the nursery school for only a few days a week, or for only half a day. Group size could therefore range anywhere from 10 to 30. Since the size of the play room remained constant, the various play sessions had a corresponding range of population densities, or degrees of crowding.

Empirical analysis of the children’s behaviour revealed clear and significant changes as the number in the room increased: more crowding led to increased aggression. In my research study, the percentage of aggressive interactions (shouting, biting, kicking, pushing) increased significantly as the room became more crowded.

The increase was more catastrophic for the boys than the girls, however, with the boys spending almost one third of their interaction time in conflicts (rising from a base of 8 per cent to a high of 32 per cent as the room became more crowded).

At least eight other published studies, controlling the level of crowding by comparing different sized rooms as well as variously sized groups of children, have now confirmed the finding that young children of nursery and primary school age were more likely to abuse and fight one another as classrooms became more crowded.

In addition to showing an increase in aggression, the ways in which the children altered their behaviour showed another dimension. The rat research had demonstrated that under conditions of crowding, some of the animals withdrew and became more isolated, whereas others became aggressive and frantically social. The same thing happened with the children in my study. There were individual differences, but the clearest demarcation in modes of response to the crowding was by gender.

The boys played with more other boys as the room became more crowded, and formed small “gangs”. But the girls’ results were in the opposite direction. They played with fewer companions in the most crowded setting. The proportion of time the girls spent alone increased from just over one-third of their time in the medium density play setting, to an astonishing near two-thirds when the room was at its most crowded.

Perhaps the longer a child stayed in the nursery, the more confident they would become, and the less they would be affected by changes in the level of crowding? Correlations between the number of months the children had attended the nursery and their reaction to the most crowded settings revealed no significant connection for the boys.

Their reactions to crowding were the same, no matter how long they had been attending the school. But for the girls, their tendency to withdraw from social contact significantly increased with the length of their attendance at the nursery.

Fourteen other scientifically controlled studies published in America and Britain have shown the same thing: that when placed in increasingly crowded conditions many children of playgroup, nursery or primary school ages engage in less social interaction, display more avoidance behaviour and exhibit more withdrawal and solitary play. But when they do interact with other children, it is much more aggressively than when the room is less crowded.

There are various theories as to why crowding has such deleterious effects on children. Of course, crowding is a psychological state, and how the crush of people is experienced will differ from one person to another, and from time to time in the same person. But it is clear that chronic crowding, characterised by excessive social stimulation, is stressful. It results in overload.

Government officers have claimed that larger classes (30 plus children) do not result in poorer tested exam performance than that of similar children in smaller classes. Yet recent laboratory studies, as well as some small-scale experiments, have shown that the experience of crowding adversely affects performance on complex mental tasks, especially those where people must interact with one another.

Concern over the narrowly academic consequences of large class sizes avoids a crucial point. Academic performance is not everything. The ways and conditions under which we teach our children have a deep impact on the kind of people they grow up to be.

The findings described here impel us to demand that the Government should urgently commission new research into the psychological effects of class size, and take seriously those aspects of our children’s school experience which lie beyond the narrow definitions of academic testing. Failure to do so could well lead us into a situation where the “road rage” of adults is matched by the “school rage” of stressed children.

Dr Brian Bates is former chairman of psychology and now senior lecturer in social psychology at the University of Sussex. For more information contact him at the School of Cultural and Community Studies, Arts Bldg, University of Sussex, Falmer, Brighton BN1 9QN.