Awareness of laterality of brain function is at least as old as written history. For example, Diocles of Carystus in the 4th century BC insightfully wrote:

There are two brains in the head, one which gives understanding, and another which provides sense-perception. That is to say, the one which is lying on the right side is the one that perceives: with the left one, however we understand. (Lockhorst, 1985)

However, Marc Dax was the first in modern times to observe a difference in function between the hemispheres. In 1836 he noticed that victims of injury to the left hemisphere (LH) but not to the right hemisphere (RH) could not speak (Dax, 1865). Paul Broca extended this work by additionally noting that often the dominant hand was contralateral to the language hemisphere (Broca, 1865).

In contrast to analog hemisphericity, the binary “hemisphericity” (hemisity) concept was more in alignment with the qualitatively different and mutually antagonistic modes of data processing of the opposite cerebral hemispheres, and certainly was much easier to quantify. Numerous “hemisphericity” reports were published (Morton, 2001, 2002, 2003a,b,c,d; Morton and Rafto, 2006). This series was continued by publication of additional “hemisity” reports (Morton and Rafto, 2010; Morton, 2012; Morton et al., 2014).

First, four independent biophysical methods were devised to separate right and left brain- oriented persons (RPs and LPs). Each of these showed a remarkable consistency in dividing large groups of individual into nearly the same groups of LPs and RPs. Based upon the identity of these hemisity subgroups, ultimately four “either-or” forced choice preference type questionnaires were created whose applications also divided a large starting group into the same RP and LP hemisity subgroups. These biophysical and derivative questionnaire methods are briefly described next.

The above new methods enabled the accurate characterization the hemisity subtype of hundreds of subjects (Morton, 2003d). This enabled MRI studies to be carried out seeking brain structural differences between LPs and RPs. Two neuroanatomical differences were found. The first was the observation that the corpus callosum midline cross sectional area of RPs was up to three times larger than that of the LPs (Morton and Rafto, 2006). The implications of this discovery will be discussed later. Second, it was observed that in 146 of 149 cases (98%) the subject's bilateral anterior cingulate cortex (ACC) in Areas 24 and 24′ was up to 50% larger on the right side for RPs, while for the LPs it was up to 50% larger on the left (Morton and Rafto, 2010), Figure 1. This result motivated the transformation of this 3 min MRI procedure into the primary standard for the determination of individual hemisity subtype, as follows:

Coincidentally in terms of the hemisity MRI findings of ACC laterality, much evidence supports the ACC being a major structural element of the brain's executive system. Remarkably, this cortical element of the ancient limbic brain region (Roxo et al., 2011), including interconnecting integrative loops (Alexander et al., 1986) between prefrontal, striatal, thalamic, and other limbic areas (Bonelli and Cummings, 2007) has repeatedly been shown to be involved in executive type activities. These include: decision making (Kennerly et al., 2006), error detection, conflict monitoring, stimulus-response mapping, familiarity, and orienting (Wang et al., 2005), response to pain and production of emotion: (Vogt, 2005), verbal and non-verbal executive tasks activity (Fornito et al., 2004), conflict monitoring and adjustments in control (Kerns et al., 2004), rapid processing of gains and losses (Gehring and Willoughby, 2002), interfacing between motor control, drive, and cognition (Paus, 2001), episodic memory retrieval (Herrmann et al., 2001) and the initiation and motivation of goal directed behavior (Devinsky et al., 1995).

Some ACC activities appear directly relevant to hemisity differences in behavioral styles. These include its participation in temperament (Whittle et al., 2008), reward and social learning (Behrens et al., 2008), expectancy and social rejection, Somerville et al. (2006), self-reflection (Johnson et al., 2006), personality (Pujol et al., 2002), will and addiction (Peoples, 2002). Even though psychoanalytic concepts were originally not intended to correspond to neuroanatomical structures, it can be noted that the ACC seems to mediate a number of different cognitive functions formerly subsumed under Freud's central element of control, the Ego. It certainly has the resources to implement the many behavioral differences between hemisity subtypes.

What is fascinating in terms of the hemisity story, is that not only does the ACC house a major brain executive element, but also that its two sides, separated by the cerebral midline fissure, are highly asymmetric. There are at least 10 reports of ACC structural asymmetries, especially in Areas 24, and 24′ which varied in an individually idiosyncratic manner, (Vogt et al., 1995; Paus et al., 1996a,b; Hutsler et al., 1998; Ide et al., 1999; Yucel et al., 2001; Pujol et al., 2002; Fornito et al., 2006, 2008; Huster et al., 2007; Palomero-Gallagher et al., 2008). Many of these reports mentioned efforts to identify behavioral consequences of these identified asymmetries, interestingly including their possible relationship to executive function, e.g., Pujol et al. (2002). However, these efforts lacked the unifying concept of hemisity.

Might this laterality of the ACC executive element provide a direct link to a subject's hemisity, thus supporting the observed relationship between the two? Indeed, it is here asserted that the discovery of the congruity of the larger side of the ACC with hemisity subtype has actually provided the missing mechanism to account for the existence of hemisity and for the differences between LPs and RPs. Further, such an “either-or” laterality context is consistent with the logic that there can be only one “Bottom-line,” “The buck stops here” executive element in any successful institutional organization, including the mammalian brain, which is completely bilateral, except for the pineal gland. Although, Descartes (1637) was logically compelled to assert this endocrine organ to be the executive “Seat of the Soul,” now, it rather appears that the executive system must be unilateral. That is, hemisity must result because an executive element, embedded in the local specialized (top-down, important details) environment of the LH, will inevitably have a different perspective than one imbedded within that of the right (bottom-up, global perspective).

Thus, the existence of major asymmetries in the ACC supports the hypothesis of the possible existence of a unilateral executive element. This idea is not new. When he learned that the bilateral ACC was the probable site of the executive system, Crick (1994) was led rhetorically to ask: “Could there be two centers of the Will?” (Sejnowski, 2004). In a “Postscript on the Will” within his book “The Astonishing Hypothesis,” (1994), Crick states that he and Antonio Damasio arrived at the same negative answer to this question by noting about the ACC that the “region on one side projects strongly to the corpus striatum (an important part of the motor system) on both sides of the brain, which is what you might expect from a single Will.” Parenthetically, neither their use of the term Will, nor the use of the term Executive System here were intended to invoke the idea of a decisional homunculus, but rather of a preconscious early response system (Libet, 1982) continually acting to optimize the survival of the organism.

With the ability to accurately determine the right or left brain individual hemisity subtype identity in hand, it became possible to answer some pressing questions: do these biophysically identified right and left hemisity subtype individuals differ significantly in their behavioral preferences? And if so, specifically how? Morton (2012) studied the behavioral responses of 150 subjects whose hemisity had previously been calibrated by MRI. He used five MRI-calibrated preference questionnaires, two of which were new. Right and left brain-oriented subjects selected opposite answers (p > 0.05) for 47 of 107 “either-or,” forced choice type preference questionnaire items. Removing overlaps resulted in 30 hemisity subtype preference differences (Table 3). These differences could be subdivided into five areas: (1) in logical orientation, (2) in type of consciousness, (3) in fear level and sensitivity, (4) in social-professional orientation, and (5) in pair bonding-spousal dominance style.

The following is an interpretation of 30 hemisity differences found: regarding Logical Orientation, LPs tended to be top-down, detail oriented, and deductive vs. RPs who were more bottom-up, big picture, and inductive. Regarding Type of Consciousness, LPs tended to be more verbal, dependent upon abstract reasoning, and oriented to find differences between objects vs. RPs who where more visual, dependent upon concrete reasoning, and able to find commonalties between objects. As to Fear Level and Sensitivity, LPs were more sensitive, taciturn, emotion-avoiding and defensive (implying a thinner barrier to fear-invoking subconscious material), while RPs were more intense, bold, talkative, emotion-embracing, and invasive. For Social and Professional Orientation, LPs were more independent, avoidant, private, and competitive, while the RPs were more orderly, responsible, open, and cooperative. In terms of Pair Bonding Style and Spousal Dominance, LPs were the less dominant spouse, who needed separateness, quietness, seeking to avoid emotionality with logic, spouse assisting, and initiator of the details of family endeavors early in the day. In contrast RPs were the more dominant spouse, needing closeness and reassurance of the other's fidelity and support while being intuitive and highly directive, ending the day by reviewing the big picture survival status of the family and making plans for the next day.

It is ironic that many of these behavioral preference differences parallel some, but not all, of the putative differences between the right and left brainers popular in folk hemisphericity (Springer and Deutsch, 1998), such as detailer vs. globalist, analytical vs. synthetic, words vs. images, abstract vs. concrete (L vs. R, here). However, many more differences were revealed, most of which as yet have no recognized brain basis, for example fear vs. confidence, or morning vs. evening, quiet vs. talkative. Perhaps the use of hemisity to identify individuals with those traits may assist in identification of their underlying brain mechanism.

As mentioned, the cross-sectional area of the midline of the corpus callosum (CCA) was found to be significantly smaller in LPs than in RPs, and to be unrelated to sex or handedness (Morton and Rafto, 2006). These observations, illustrated in Figure 2, have had several ramifications. To begin with, if the executive element of the anterior cingulate was in the same hemisphere as language, as is the case for most LPs, there would be less need for transcallosal communication than if the executive element was located in the opposite non-language hemisphere. Thus, the CCA in LPs would be predicted to be smaller than in RPs, as observed.

Further, hemisity behavioral outcomes contradict several commonly held beliefs about sex and the brain: first, the hemisity results lay bare the underlying basis of the previous controversy about gender and laterality. The confusion occurred because in all earlier CCA studies, the hemisity of the subjects was unknown. This caused an unwitting confounding of the results for subjects sorted only by sex or handedness with hemisity, a major factor influencing CCA (Morton and Rafto, 2006). This error brings into question the common view that the male brain is more specialized due to its higher laterality (McGlone, 1980). Rather, the CCA data strongly suggest that it is the left brain-oriented individuals of either sex who are more lateralized as a class than males are. Correspondingly, right brain individuals of either sex are less lateralized and more broadly generalized as a class than females are, thus contradicting another sexual stereotype.

Second, these findings appear to end the controversy about which sex has the larger corpus callosum (Luders et al., 2003). There was no significant difference between the two sexes in either their mean CCA, its size range, or in the IQ of the subjects (Morton and Rafto, 2006). Rather, the two largest CCAs of individuals from among our 113 subjects were possessed by a right brained female and by a right brain male (10.1 and 9.2 cm², respectively). Conversely, the two smallest CCAs were 4.8 cm² for a left brained male and 4.5 cm² for a left brained female. All four of these individuals held doctoral degrees and professorial status.

Third, lack of awareness that hemisity contributes to CCA makes it probable that the European studies reporting mean CCAs for males to be larger (Clarke et al., 1989) and American–Australian studies, showing larger female mean CCAs (Holloway et al., 1993) were both correct. Their disagreements could well be based upon regional population differences in hemisity, an important but uninvestigated topic.

Fourth, it is becoming clear that members of either sex with the same hemisity have more behavioral traits in common than do same sex individuals of the opposite hemisity. This is strongly supported by data from the MRI calibrated preference questionnaires (Morton, 2002, 2003c, 2012). Thus, it would appear that several hemisity traits are presently being misidentified as male or female sex traits. That is, men in general do not “hide in their caves of silence” (Tannen, 1990; Gray, 1992). In fact, in contrast to their right brain counterpart, left brain-oriented females are every bit as “private” as left brain-oriented males (Morton, 2002, 2003c, 2012). Similarly, females do not always “rule the roost.” It is the right brain-oriented person who tends to dominate the nuclear family, be they male or female (Morton, 2002, 2003c, 2012). Because of the newness of hemisity and its new behavioral distinctions, sex traits have never been studied together with hemisity traits. Books such as John Gray's “Men are from Mars, Women are from Venus” (1992) appear to fit perfectly for about half the population (~60%), that is, for the RFs and LMs. The other half (~40%) say it is totally alien to them. However, if the pronouns are reversed from “him” to “her” and vice versa in the book, then the other half of the population (RMs and LFs) strongly identify with it (Morton, unpublished). So it appears not to be a description of sexual differences but rather of hemisity differences. Thus, the recognition of the quantifiable existence of hemisity can bring new clarity to human behavior.

Morton (2003d) investigated the distribution of hemisity subtypes within the general population. It was proposed (Morton, 2003d) that in an unsorted population not only would the numbers of male and females be equal, but that the numbers of RPs and LPs would also be similar. It was hypothesized that hemisity sorting in populations would only occur after admission into a school or an organization where entrance was competitive and selective. In the US, this typically first occurs at the university level because in essentially all public elementary, high schools, and even some community colleges, essentially no applicants are excluded and all must complete a similar general core curriculum in order to graduate.

Morton et al. (2014), using the Best Hand Test (Morton, 2003b) and the Polarity Questionnaire (Morton, 2002), measured the hemisity of 1049 public high school upper classmen from Hawaii and Utah. As predicted, in this sample there were similar numbers males (n = 522) and females (n = 527), and of right (n = 526) and left (n = 523) brain-oriented individuals. There were reciprocal complementary relationships between right males (RMs, 39%, n = 206) and left females (LFs, 40%, n = 210), and correspondingly among left males (LMs, 61%, n = 316) and right females (RFs, 60%, n = 317), thus confirming the non-sorting hypothesis. This suggests that females are slightly enriched in RPs and males are with LPs, and therefore that the average CCA of females should be slightly greater than of males. However, these differences do not appear to obviate the four generalities of the preceding section.

The equalities of hemisity within the general population were lost among 228 competitively selected college freshmen, 57% of whom now showed left hemisity. Students in more specialized upper- division classes (Morton, 2003d) showed an increased range of hemisity distributions, from 35% left brained individuals in a civil engineering seminar to 68% left brained persons in a home economics course.

Even more pronounced hemisity distribution differences were found in university representatives of 17 different professions, ranging from only 21% left brained among astronomers and 33% left brained among architecture professors, to 83% among biochemistry professors and 86% among microbiology professors (Morton, 2003d). Professional librarians (n = 15) were predominantly left-brained (73% LPs), while academically trained musicians (n = 91) including concert pianists (n = 47) were predominantly right-brained (32% LPs) (Morton et al., 2014).

Within professional groups there were differences related to area of specialization. For example, among practicing civil engineers, only 39% of design civil engineers were left brained, compared to 74% of construction civil engineers. Morton (2003d) suggested that individuals in primarily “top-down” professions working at structural levels that are subdivisible, such as microbiologists, biochemists, and particle physicists, were more left brained. In contrast, those in more “bottom-up” macroscopic or gestalt-oriented professions such as architecture, civil engineering design, and astronomy, tended to be more right brained. Thus, as it may be seen, hemisity appears to play a profound role in career development.

An explanation has been proposed to account for the sorting of hemisity in higher education and career selection (Morton, 2003d). That is, sorting occurred as the result of RPs and LPs doing what they liked best. Topics at which each excelled relative to the other resulted in one hemisity subclass doing well or poorly compared to the other. Rewards from success, difficulty, or failure shaped individual opinion of the liking or dislike of specific topics. This led to the selection of topics bringing personal success and to the avoidance of those bringing failure. Thus, in general, it appears that one ends up being an architect or microbiologist simply by doing what one enjoys most.

Although both the Best Hand Test (Morton, 2003b) and the Polarity Questionnaire (Morton, 2002) were used in the above population studies, the viability of using the more easily administered Polarity Questionnaire alone to determine the hemisity of large groups was considered by comparing its outcomes here with those of the Best Hand Test alone (Morton, 2003b). For a high school population (n = 703), the outcomes of the two methods differed in only 5.6% of cases. Further, the Polarity Questionnaire was able to assess the hemisity of the 10.4% individuals whose Best Hand Test results were indeterminate. This supported the idea that, not only are the two measures complimentary, but also that perhaps future studies using the Polarity Questionnaire alone, or in combination with one or more of the other calibrated hemisity questionnaires might be acceptably accurate for the estimation of hemisity of large English speaking populations. However, the extreme outcome sensitivity to wording of Polarity Questionnaire statements (Morton, 2002, 2003c) suggests that great care must be taken in its translation into other languages and cultures. In contrast, biophysical hemisity methods, such as the Best Hand Test, while much more demanding to assess, appear to be language and culture independent.

Because the grading of the Best Hand Test, a research instrument, is complex, technical, and time consuming, it is not practical for use in general hemisity studies. As indicated above, similar results are easily obtained by the Polarity Questionnaire. Further, it has been shown that combined use of the Polarity Questionnaire with the three other rapid binary hemisity questionnaires that have been developed: the Asymmetry Questionnaire (Morton, 2003c), the Binary Questionnaire (Morton, 2012), and the Hemisity Questionnaire (Morton, 2012), enhances the 80% certainty of the hemisity subtype result of a single questionnaire to about 95% for combined use (Table 2.) Each questionnaire takes only a few minutes to administer and grade.

Six useful conclusions are among many that can be derived from this review of hemisity: (1) Research now supports the view that the existence of hemisity is inevitable, due to the unilateral nature of a structural element of the executive system. (2) Quantitative methods have been developed to make it possible to assess any person in terms of their probable right or left brain orientation. (3) A primary standard has been discovered that enables the absolute hemisity of an individual to be determined, based upon anatomical landmarks within the brain. (4) A number of the many “either-or” traits that separate the cognitive and behavioral styles of RPs and LPs have been identified, most of which as yet have no known ties to brain asymmetry. (5) Methods now exist which can determine the average hemisity of groups with considerable sensitivity. (6) The recognition of the quantifiable existence of hemisity as a second dyadic personal identifier after sex can bring new clarity to human behavior.

The neuroanatomical differences between left- and right-brain oriented individuals raise the question of how these features develop. Correlating parent and offspring hemisity types might provide first insights into the development of this phenomenon. However, extensive genetic research will most likely be necessary to fully unravel the development and implications of hemisity.