FLOWERS WORLD

Theoretical Explanations
Given that there is little theory to guide us, what explanations might we present for these findings? We suggest that the explanations fall into three categories: (1) simple learned associations of flowers with positive social events, (2) associations of flowers with food that could be part of an evolutionary response promoting food search, and (3) flowers as specially evolved human sensory mood enhancers. We will review the evidence and rationale for each possibility.
Learned Associations. We cannot eliminate the strong possibility that the global symbolic meaning of the flowers leads to a positive learned response. Our attempt to use men as a test of differential learning of positive responses to flowers may not be sufficient. It may be necessary to find a society or group of people who do not use flowers in any symbolic way, if possible. Since the mere presence of flowers in study two did not elicit positive social behavior, it remains possible that the learned association between flowers and giving is accounting for much of the effect. The design of study two apparently created an expectation in participants that they would receive a flower. When they did not, the disappointment was palpable. A research design that looks at the effect of flowers in pubic spaces, or perhaps along roadsides, where there is no expectation that they will be given to individuals might answer this question. But even if it is the case that the effect is purely an associative one, it is still somewhat surprising that there are such strong positive mood effects and secondary benefits simply following the receipt of flowers. This associative explanation evades the question about how flowers in comparison to other positive objects, such as food or other highly desirable gifts, have come to have stronger positive emotional associations across cultures and history.
Evolutionary food association. Rather than hypothesize a simple learned association of flowers and positive social habits, one might hypothesize that there has been some evolutionary advantage to the attraction of flowers in their relation to fruits and other food products. For example, Orians and Heerwagen (1992) suggest that aesthetic preferences for landscapes and potentially other growing things are related to early hominid survival when these environmental cues would be related to foraging success. Thus, flowers would be preferred because humans became hard-
wired to “emotionally” appreciate “beauty” associated with food gathering.
Orians and Heerwagen (1992) postulate that flowers would evoke a positive response (joy), that predicted future food supplies and possibly a better place for rearing progeny.
Pinker (1997) makes a similar suggestion that the attraction to flowers is hardwired into our brain because flowers directly signal the future availability of fruit, nuts, or tubers. Humans remember where and when flowers were observed to obtain food in the future. The strength of this argument is mitigated by the fact that mammals find food with all their senses, most of which are of more immediate use than long term memory for key environmental features (Stiles, 2000). More to the point, the showy flowers that humans seem to find especially attractive generally do not produce edible fruit.
Instead of the association being either learned or innate, we could take a middle position that humans are biologically primed to associate flowers with happiness. Under this condition, recognition of flowers would not be hard-wired, per se, but aspects of flowers would easily become associated with happiness and it would be very difficult to associate flowers with negative emotions (Cook and Mineka, 1987).
Mood Enhancers. Our last explanation takes a different approach but is in some ways an extension of the middle position. The third hypothesis is that various sensory elements of flowers combine, even serendipitously, to directly affect mood. The effect would occur even if a flower were a de novo event in a human experience. Using this explanation, flowers prime positive psychological responses because they are “super stimuli,” directly affecting moods through multi-channel sensory interactions (or one of the channels alone might be able to carry the effect). It would be easy to learn positive associations, but the general mood effect would be predicted to occur without learning.
Most of the sensory attractors lead partially or entirely to changes in mood. This process places moods or emotions in a central position for the evolution or even co-evolution of plants and people – a process that is useful for the results of our studies. This theory is congruent with Panksepp’s (2000a) concept of an affective consciousness. According to Panksepp, there are primal neural networks in the posterior thalamic, tectal and periaqueductal regions of the mesencephalon that constantly process emotional information. Organisms seek this information. Sensory stimuli such as visual symmetry, color, odor, and pheromones provide the information sought and affect moods. We will briefly examine some examples of these sensory stimuli as they are not often used to explain socio-emotional results.
With respect to symmetry, Enquist and Arak (1994) argue that we have an evolved preference for patterned symmetries because these can be detected easily as a recognizable signal within in a wide variety of visual arrays. In other words we are attracted to symmetry. The ease of recognition and the familiarity engendered should be associated with improved mood, as well as easy identification (see Zajonc, 1980; for the rationale for an association of familiarity and emotion). There does not need to be an established memory or association to the flower in order for there to be a positive effect. It is the symmetrical pattern alone that is important. One might look at it from the point of view of the flowering plant and describe the effect as one in which the plant uses symmetry to attract a human for seed or tuber dispersal or to protect the plant from predators or dangerous environments. Ultimately, then the plant that only has a preferred symmetry, even if it has no other product, potentially can be protected and dispersed by humans. The result for humans is improved mood.
Similarly, we may have preferences for certain colors based on the primate trichromatic color visual system. Such a preference might have evolved because the various color channels are important in finding ripe fruit against a green background (Osorio and Vorobyev, 1996; Parraga, Troscianko, and Tolhurst 2002) or in distinguishing high protein leaves (Dominy and Lucas, 2001). Again, the preferences for particular colors may be separated from their use in spotting food and become rewarding more generally. Plants with preferred colors that have no other products would be protected and dispersed. Plants with preferred colors and symmetry may become “superstimuli” with the combination.
We might also have a preference for specific floral odors. To our knowledge this has not been extensively studied within the psychological literature for perception and sensing although perfumers have shown that differences in preferences exist (see Jellinick, 1997). Obviously, humans use olfactory information and relate this to other sensory information. The coordination of olfactory and visual cues is known to influence neurological responding. For example, Sarfarazi et al (1999) showed that the amplitude of the P400 visual event related potential was decreased if the odor cue did not match the visual cue. Considered alone, specific odors seem to modulate moods (see Baron, 1997, Schiffman et al., 1995; Shiffman, Suggs and Sattley-Miller, 1995; Freyberg, Wilson, and Haviland-Jones, under review). Schiffman and her colleagues have shown that popular colognes that often have a floral topnote will reduce depression, and Freyberg, Wilson, and Haviland-Jones have shown that certain putative pheromones or fragrances can reduce negative moods, for example.
There is also the possibility that humans might be sensitive to the effects of floral social chemicals. We have long known that a variety of species are responsive to pheromones produced by plants to mimic sex pheromones. The perfume manufacturers have believed for centuries that humans are sensitive to such pheromones and use pheromonal animal products as the base for fine perfumes (e.g., civit, musk). Several researchers have demonstrated mood effects of androstodienone on women (Jacob and McClintock, 2000; Wilson et al., under review; Freyburg, Wilson, and Haviland-Jones, under review). Such social chemicals might function in courtship and increase social behaviors, as well as affecting moods.
Plants are sometimes considered to be biological chemists. Their chemical make-up is rapidly responsive to other species, time of day, and other variables. They could certainly use chemical or visual cues to attract humans. Following the argument that plants have significant, largely unexplored chemical potential, Wilson The natural products of plants and animals are a select group in a literal sense. They represent the defense mechanisms and growth regulators produced by evolution during uncounted generations, in which only organisms with the most potent chemicals survived to the present time . .. .Very few pharmaceuticals have been invented from a knowledge of the first principles of chemistry and medicine. Most have their origin in the study of wild species. . .( p. 134).
Is it reasonable to propose that plant-human co-evolution or even domestication is based on socio-emotional rewards? There are many instances of such plant and animal relationships, though they are not usually thought of in this way. Plants may reward animals for defending them. For example, the “swollen thorn” acacia trees of Central America have large thorns which can be used for ant nests. Their leaves have nectaries, which produce nectar consumed by the ants. In return the ants attack any herbivores and even remove vegetation around the trees. If the ants are removed, the plants are soon killed by predators (Janzen, 1966).
Plants attract animals for a variety of reasons and by a variety of methods. Plants utilize animals for pollination, seed dispersal and protection. While the vast literature on the attractiveness of flowers has been focused almost entirely on insect pollinators, some flowers attract vertebrate pollinators. For example, a number of bat-pollinated flowers emit a sulfur-like odor that mimics odors used in bat mating and social recognition (von Helversen, Winkler, and Bestmann, 2000). At least one flower, Mucana holtonii, reflects and focuses bat sonar signals to attract pollinators (von Helversen and von Helversen, 1999). Other flowers attract hummingbirds with color; such flowers tend to be red, have symmetrical tubular flowers, and provide a heavy nectar flow.
A wide variety of plants utilize vertebrates as seed dispersing agents (Stiles , 2000). Plants have a variety of powerful mechanisms that could affect mood positively and attract animals for seed dispersal -- including color, odor or even sound. For many plants, including flowering plants, humans are the primary seed-dispersing organism. To our knowledge, humans are the only organism that routinely digs up, divides and replants tubers, bulbs and corms of flowers. Some domesticated flowers may have become dependent upon humans for propagation (Comba et al., 1999). For example, cultivated orchids are a highly selected and preferred flower that is hand pollinated by humans. Orchids have a sensory attractiveness but little or no food or medicinal usefulness for people. These scattered features continue to suggest that plants can attract people even if there is no reward in terms of food, medicine, shelter and so forth. Plants may only enhance or prime positive moods.
The idea that flowering plants with no known food or other survival value have coevolved with humans by using an emotional niche spawns a couple of predictions that can be addressed in future research. First, domesticated flowers should, in general, be better at inducing positive emotions than their progenitors. Second, different flowers will induce their effects through different combinations of modalities. Some might be primarily visual. Others might be visual and olfactory. Some may even mimic human pheromones. Given the wide HLA variation among humans in responses to odor (Jacob, et al., 2002), it is also likely that humans vary in their responses to particular flowers. There may be gender and ethnic effects as well.
An extensive literature search for research on why certain flowering plants are selected for domestication or propagation yields almost nothing. Many books and articles discuss the domestication of plants useful for humans in food, medicine, shelter and so forth; the notable exception is the domestication of flowers. We suspect this is part of the general neglect of emotional processes as major contributors to biological evolution. Flowers cultivated by humans occur in the wild in disturbed ground. Usually they are weeds taking cultivated land away from edible/burnable/constructive produce. If the flowers induced positive emotions they might have been allowed to remain in or near the cultivated fields. The loss in food production due to weeds would have been offset by an increase in positive emotion. The selected offspring of these pleasing plants might have become even more pleasing. We hypothesize that as flowers moved into the new niche created by agriculture there was an increase in variation and the more pleasing and attracting flowers were allowed to remain. At some point humans might have moved from merely tolerating these weedy species to actively saving and sowing the seeds. It has not escaped our attention that the scenario we present for the evolution of flowers is very much like the scenario that Budiansky (1994) presents for the evolution of dogs. Flowers may be the plant equivalent of companion animals.
Our hypothesis is that cultivated flowers fit into an emotional niche - their sensory properties elicit human positive emotions. The flowering plants are thereby rewarding to humans and in return, the cultivated flowers receive propagation that only humans can provide. Demonstration of such a phenomenon fills several gaps in the literature. It supports the basic significance of emotion for survival. As a corollary it supports the adaptive function of positive as well as negative emotion. Finally it opens an area of investigation into the psychological relationships between humans and other species through their sensory properties that have been relatively neglected.