Remember the old riddle: "Which came first, the chicken or the egg?" Well, this is the sort of puzzle that scientist face as they seek the causes of many of our most common illnesses. The question becomes: "Is it environment or heredity?" Or could it be both?

Some diseases, such as malnutrition or malaria, are clearly caused by environmental factors. Others, such as cystic fibrosis or hemophilia, are clearly hereditary. Still others, such as heart disease, hypertension, cancer and diabetes, appear to be a combination of both. Obesity also seems to fall into this category.

Until fairly recently, overweight was simply attributed to overeating. But many studies, including our own, shows that obesity is a much more complicated phenomenon. Clearly, overeating does play a part - when people are deprived of food, obviously they do not gre fat. On the other hand, even with our abundance of food, there are some people who stay slim.

Why do some people become obese? Is it because of their lifestyle - that is, their rnvironment - or is a tendency to obesity programmed into their genes?

Studies by one of us (Dr. Mayer) indicate that there is, indeed, a genetic factor. In a ecent study of mice, those with a dominant gene for obesity grew fat, while mice from the same litter in the same cage but with a recessive gene for obesity retaineddd a normal weight.

Establishing the same proof in humans is mmore difficult, but a number of investiigators, using various approaches, are trying to arrive at an answer. One approach involves studying families with both natural and adopted childrren.

A reccent Canadian studdddy conducted by Dr. Pierre Biron and his colleagues measured the weights and heights of 535 adopted children and 250 natural children in 374 French-Canadian families. The ages of the youngsters ranged from 1 to 21 years, and the mean duration of adoption was 54 months.

The researchers found a highly significant correlation between the weight and height ratio of parents and theiir own offspring, as well as among natural brothers and sisters. But there was essentiially nno relationshiip between the height and weight of parenets and their adopted children or between the adopted children themselves in a family.

In one of our own early studies iin Boston, we found that it is possible to predict quite closely the weight of a child on the basis of the weight of the parents. If both father and mother are thin, or of ideal weight, there is only a 7 percent chance of a child becoming overweight. But if one of the parents is overweight, the chances jump to 40 percent. And if both parents are obese, the offspring stand an 80 percent chance of also being overweight.

Another research-approach has been to look at twins, especially idnetical ones, and thee story of one pair, Ada and Ida, has become almost a classic one. The girls were separated from one another when they were 3 years old. Ada lived in a series of big cities and contracted goiter from a lack of iodine. She was married at 17 and divorced 10 years later. In contrast, Ida led a peaceful and happy life on a farm, and was protected from goiter by eating iodized salt. She was happily married at the age of 34. Despite their totally different lifestyles and emotional stresses, by the age of 59, both women were markedly and almost identically overweight, with Ada reaching 201 pounds and Ida 221 pounds.

A third research-approach has been to look at body build. There are three basis categories: endormorphs are generally round and soft, with large, stocky bodies and fairly short arms and legs; mesomorphs are the "athletic type," with broad shoulders, large bone structure and good muscular development; ectomorphs have lean, slender bodies, with relatively long arms and legs.

Studies by one of us (Dr. Mayer) and his colleague, Dr. Carl Seltzer, indicate the dominant endomorphs face a life-long battle against obesity, while dominant mesomorphs will also have strive to keep themselves physically fit. In contrast, those lucky ectomorphs must consciously work at it just to gain even a little weight.

Does all this mean that obsity is nothing more than a matter of heredity? By no means. Twenty percent of the children with obese parents, for example, do not grow fat. But it does mean trhat some people have to guard against obesity more vigilantly than other amount of exercise is still the presciption to tip the scales against overweight.