References

Ricco, A. (1877). Relazione fra il minimo angolo visuale e l'intensita luminosa. Memorie della Regia Accade-mia di Scienze, Lettre/Arti in Modena, 17, 47-160. Piper, H. E. (1911). Uber die netzhautstrome.

Archiv der Anatomisch und Physiologie, Leipzig, 5, 85-132.

RIGHT-SHIFT THEORY. In the research area of handedness (cf., Corballis, 1997) and brain function, Marian Annett (2002) proposes her right-shift theory (R-ST) that attempts to explain the relationship between left- and right-handedness, and left- and right-brain specialization. Statistically, approximately 90 percent of people in the general population demonstrate right-handedness, i.e., left-hemisphere dominance for motor control. R-ST suggests that handedness in humans and nonhuman primates depends on chance but that it is weighted/biased towards right-handedness in most individuals due to a variable called "right-hemisphere disadvantage." R-ST argues for the existence of a single gene that accounts for a "right-shift" tendency that evolved in humans for the purpose of facilitating the growth of speech in the left hemisphere of the brain. Thus, R-ST describes a right-shift factor for left-cerebral dominance. R-ST has implications for a wide diversity of issues concerning human abilities and disabilities, including both verbal and nonverbal intelligence, spatial reasoning, dyslexia and educational progress, mental illness, and performance and skills in sports activities. In another line of reasoning concerning handedness, Corballis (2003) observes that the strong predominance of right-handedness appears to be a uniquely human characteristic, whereas the left-cerebral dominance for vocalization occurs in many species, including birds, frogs, and mammals. It is speculated that right-handedness may have emerged because of an association between manual gestures and vocalization in the evolution of language; Corballis argues that language evolved from manual gestures, which gradually incorporated vocal elements. Such a transition presumably arrived via changes in the function of Broca's area in the brain [i.e., the cerebral area, named after the French phy sician Paul Broca (1824-1880), that is involved in the production of speech] where in monkeys its homologue has nothing to do with vocal control, but contains the so-called mirror neurons that embody the production of manual-reaching movements as well as the perception of the same movements performed by others. The mirror neurons system is bilateral in monkeys, but mainly left-hemispheric in humans where it is involved both with vocalizations and manual actions. Based on evidence that Broca's area is enlarged on the left side in Homo habilis, it is suggested that a link between gesture/action and vocalization may go back about two million years, even though other evidence indicates that speech may not have become fully functional or autonomous until Homo sapiens appeared on the scene about 170,000 years ago. See also DARWIN'S EVOLUTION THEORY; GAL-TON'S LAWS; GENETICS, LAWS OF; LANGUAGE ACQUISITION THEORY; LANGUAGE ORIGINS, THEORIES OF; LATERALITY THEORIES; MENDEL'S LAWS/PRINCIPLES; MIRROR NEURONS THEORY; SPEECH THEORIES. REFERENCES

Corballis, M. C. (1997). The genetics and evolution of handedness. Psychological Review, 104, 714-727. Annett, M. (2002). Handedness and brain asymmetry: the right-shift theory. Philadelphia, PA: Psychology Press. Corballis, M. C. (2003). From mouth to hand: Gesture, speech, and the evolution of right-handedness. The Behavioral and Brain Sciences, 26, 199-260.

RINGELMANN EFFECT. See SOCIAL LOAFING EFFECT.

RISK AVERSION EFFECT. See DECISION-MAKING THEORIES.

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