Introduction: The Genetic Algorithm.

Since their conception in the mid 60’s (see Holland 1992), evolutionary algorithms have provided many novel and interesting solutions to problems in a wide variety of domains. From the design of turbine blades and simple autonomous robots to the optimal geographical placement of a series of gas filling stations, the fundamental techniques involved in all these examples are identical and are based on nature’s own design strategy (Goldberg 1989). In a very simplistic sense this is survival of the fittest. Many different entities are created and vie for the chance to reproduce and so pass on their genetic information. Thus more successful genetic information is created whilst less successful solutions are discarded. The most popular and successful type of evolutionary algorithm is currently a ‘Genetic Algorithm’ (GA).

There are 3 main areas of interest when developing a GA; 1) How to map entities onto gene strings (encoding);

2) How to calculate the successfulness of a solution (fitness); 3) How to allow the combination of solutions (breeding).

Encoding of problem space can be achieved in many different ways, but in essence most GA’s use a simple ‘alphabet’ from which ‘words’ can be made up which describe an entity. Roughly speaking, in human DNA this alphabet consists four different nucleotides: adenine, thymine, cytosine, and guanine (A,T,C,G). These are put together to make a long word (our gene string, genetic code or genotype) which fully describes our individual physical make-up. An individual’s genotype is the product of millions of years of evolution, and is created as a combination of the parents’ code. Which code is ‘good’ and which code is not is defined by a ‘fitness function’. In natural evolution, the fitness of an entity can only be gauged with hindsight and relates to the successfulness at becoming an ancestor. In a GA the fitness function is used to evaluate how well an item fulfils a specified selection criteria. This is then used to define how many offspring the entity will produce. In these studies investigations are being conducted about how various attributes of bottle design relate to perceptual judgements about that bottle. In this case, the fitness function is related to what percentage of people sampled have a preference for an individual stimulus bottle and is used to preferentially enhance the appearance of the genetic code for these likable traits in future bottle generations.

Finally, what does 'breeding' mean? How can bottles possibly breed!? Once a problem domain (shampoo bottles) has been encoded into a simple alphabet (in our studies we used just two ‘letters’ 0 and 1) an initial ‘random’ population can be created by creating an initial gene pool (random strings of a specified length of O’s and I ’s) and generating the bottles which these strings specify. For two bottles to breed, one simply takes a portion of the genetic string from one bottle and the rest of the genetic string from the other bottle and combines the two (a percentage of the letters are changed randomly to simulate mutation) and the bottle generated which the resultant string

specifies. Mutation is important since it helps stop stagnation. If there is not enough diversity in a population it may not be possible to create any improvements. By randomly mutating a percentage of the letters, new areas of the problem space can be explored and the population is less likely to get stuck at a local maximum.

The application of evolutionary algorithms to optimise aesthetic values is still in its infancy and there is limited literature on the subject.

In a study of Mondrian’s art, McManus etal (1993), tested whether an evolutionary technique could be used to allow subjects to create their own Mondrian style paintings. A first study demonstrated that subjects preferred Mondrian’s paintings to those created by the computer (showing that consistent preferences did indeed exist). A second study used a hill climbing approach, whereby a small random modification to the images was made (a line moved up or down, left or right) and the painting rejudged. If the alteration improved the quality of the solution the alteration was kept and a further modification made; otherwise the alteration was discarded and a

Figure 3.1: Mondrian (1928) -

“Composition with Red, Yellow different one tried. The idea bemg that subjects

and Blue would select pictures that they preferred (from a

generation of 25 pictures) and evolve their own optimal piece. It was expected that the final composition would contain the most pleasing arrangement for a specific subject. Neither consistency between the final paintings was found, nor was there directional preference towards Mondrian’s sense of proportions. In this case it was not possible to evolve preferred shapes.

There are several possible reasons for the failure of the McManus et al study. Perhaps the algorithm used was not suitable to the problem. A better solution may have been to use a GA. Alternatively it may simply be the case that the reason Mondrian’s sense of proportions were so popular in the first study was because the subjects had been

previously exposed to them. Indeed current priming studies show that priming can occur months, even perhaps years before a recognition event and still have an effect (Bruce 1994). Bruce’s studies have shown a mere exposure effect where priming can occur from the posters advertising for subjects to take part in the experiment. As has been previously noted, advertising plays an important role in determining what concepts consumers associate with what product. For this reason it is not possible to use existing shampoo bottle designs in an experiment to find out what concepts are associated with which design. An entirely novel stimuli set needed to be created. One that would mimic the variability inherent in the current range of shampoo bottles, but one that would not be so reminiscent to instantly remind one of a certain brand. A study conducted by Johnston and Franklin (1994) allowed subjects to evolve attractive face shape using a GA. The technique used is rather crude and involves ‘cutting and pasting’ a variety of facial components together (in a configuration specified by a gene string). The computer first generated a small population of

‘random’ faces. Each of the populations of faces was rated by a subject for beauty and the fittest faces were bred together. The process was reiterated for several generations until the change in fitness from one generation to another was negligible. The faces evolved by subjects using this methodology were consistently different in shape from average. This supports the findings of Perrett et al (1994) that the concept of facial beauty is not ‘in the eye of the beholder’ but rather involves shape attributes about which subjects agree.

3.4 Aims.

There are two aims of the study:

1) The assessment of whether subjects agree on perceptual attributions.

3.5 Genetic Evolution of Bottles Experiment.