Introduction

We move now from a concentration on Delta’s foundational theoretical concepts to an exposition of Delta’s theory of practice. This may be likened to a shift from “science” to “engineering”; that is to say, the scientific aspect of Delta Theory, which is founded on general features of Homo sapiens regardless of persons and settings, now shifts to the practice aspect, which treats the variation of persons and settings as it affects the delivery of efficacious influence. Everyone who has purposed changes in others knows that every instance requires actions particularized to the situation. Every engineer who has designed a bridge did so by applying the principles of physical science to the infinitely variable local conditions of soil and stream.

The Spread of Obesity

Consider the study “The spread of obesity in a large social network over 32 years” (Christakis & Fowler, 2007). Analyzed with the concepts and mathematics of social network theory, the results created a stir not only in health and social science circles, but also in the popular press. Social distance (degrees of separation) plays a stronger role than geographical distance in the spread of behaviors or norms of obesity. For example, no effect was observed on ego if an immediate neighbor became obese. But if an ego stated that an alter was a friend, ego’s chances of becoming obese increased by 57 percent if the alter became obese. Between mutual friends, ego’s risk of induced obesity increased by 171 percent; however, if only the alter and not the ego declared friendship, there was no statistical relationship. Among pairs of adult siblings, one sibling’s chance of becoming obese increased by 40 percent if the other sibling became obese, but the effect is gender specific: Among brothers, an ego was 44 percent more likely to follow obesity; among sisters, an ego’s chance of following a sibling into obesity increased by 67 percent. Husbands and wives affect each other similarly in following the spouse into obesity (husband egos 44 percent, wives, 37 percent). The authors are persuaded that the mediating factor is a change in norms, not a direct imitative phenomenon such as diet change.