Gene Mazzola's paper

Handedness and Hair Whorls

by Gene Mazzola

 

     Statistically, ca. 92% of humans are right-handed and ca. 8% non-right-handed or ambidextrous to various degrees.  A little-known relationship exists between right/left-handedness and the direction of rotation of a person’s hair whorl.  The author’s interest in this topic developed from an interest in stereochemistry and the “handedness” of many molecules.

 

     When a relationship between handedness and hair-whorl direction was initially proposed, it was suggested that a direct correlation existed between the two phenomena, viz.:  the hair whorls of right-handed people swirled in a clockwise direction while those of left-handed persons went in a counterclockwise direction.1   However, this appeared to be a possible oversimplification because hair whorls must necessarily rotate in one direction or another while handedness is more of a gradation from completely right-handed, to predominantly right-handed, to ambidextrous, to predominantly left-handed, and to completely left-handed.

 

     The correlation between handedness and hair whorl direction has been studied to some degree.2-5 It should be noted that the following explanation is still a simplification in that more complex genetic anomalies, such as internal organs appearing on the opposite side of human bodies compared to where they are usually located, e.g. hearts on the right side, have not been considered in the following discussion.  Additionally, partially and fully ambidextrous individuals have been included with left-handed people in the “non-right-handed group (NRH)” in an explanation of this phenomenon:

 

(1.) The biological factors that govern handedness also affect hair-whorl direction.  Alleles are pairs, or series, of genes that can be dominant, or recessive, in determining hereditary characteristics.  Organisms typically have two alleles for a specific trait, one being inherited from each parent.

 

(2.) Dominant alleles dictate both right-handedness (RH) and a clockwise (CW) hair spiral.  Having even a single copy of a dominant allele yields a right-handed person, with a clockwise hair whorl, as is the case for most people.

 

(3.) Recessive alleles individually dictate clockwise or counterclockwise hair-whorl direction and also handedness preference.  When both copies of the allele are recessive, there is a 50:50 chance of a person (i) being either right- or non-right-handed and (ii) having a clockwise or counterclockwise hair whorl.

 

     Thus, if a person has a counterclockwise-rotating hair whorl, that individual must have two copies of the recessive allele but has only a 50% chance of being non-right-handed.  Similarly, only half of the non-right-handed people, who again carry two recessive alleles, will have counterclockwise hair whorls.  Looking at the population as a whole, the following patterns emerge:

 

(1.) People who have a dominant hair whorl-handedness allele are right-handed      (RH), have clockwise-rotating (CW) hair whorls, and comprise ca. 84% of the      population.

 

(2.) People who have two recessive alleles (and thus no dominant allele that would produce Case #1 above) exhibit the following characteristics and comprise       ca. 16% of the population:

 

     (A.) Half of those will be right-handed (RH, 8%)

            (1) Half will have clockwise-rotating hair whorls (RH/CW:  4%)

            (2) Half will have counterclockwise-rotating whorls (RH/CCW:  4%)

 

     (B.) The other half will be non-right-handed (NRH, 8%)

            (1) Half will have clockwise-rotating hair whorls (NRH/CW:  4%)

            (2) Half will have counterclockwise-rotating whorls (NRH/CCW:  4%)

 

     Thus, the hair whorls of ca. 92% of the population rotate in a clockwise direction:  (i) all of the dominant-allele, right-handed people (#1, ~84%), (ii) half of the recessive right-handed people (2.A.1, 4%), and (iii) half of the non-right-handed people (2.B.1, 4%).  The hair whorls of only ca. 8% of the population rotate in a counterclockwise direction:  (i) the other half of the non-right-handed people (2.B.2, 4%), and (ii) half of the recessive right-handed people (2.A.2, 4%).

 

     Right-handed persons thus comprise 92% of the population (Groups 1-23 below).  Persons with a clockwise-rotating hair whorl have an 88% chance of being right-handed [84% (#1 above) + 4% (2.A.1)].  However, individuals whose hair whorls rotate in a counterclockwise direction have a 50% chance of being non-right-handed [#2.B.2].

 

References:

(1.) H. Brunner, Right or Left in Nature and Elsewhere, Wiley-VCH (1999).

(2.) A. J. S. Klar, Genetics, 165, 269-276 (2003).

(3.) A. J. S. Klar, Genetics, 170, 2027-2030 (2005).

(4.) C. Brodie, American Scientist, 92, 27-28 (2003).

(5.) A. A. Beaton and G. Mellor, Laterality, 12, 295-301 (2007).

 

     The table below summarizes the above information for 25 groups of people (in increments of 4 percent with 4 persons in each group) regarding the occurrences of handedness and hair-whorl direction in a random collection of 100 people. 

 

 

Handedness-Hair Whorl Correlation Percentages

RH

CW

Group 1

(4%)

RH

CW

Group 2

(8% total)

RH

CW

Group 3

(12% total)

RH

CW

Group 4

(16% total)

RH

CW

Group 5

(20% total)

RH

CW

6 Group

(24% total)

RH

CW

Group 7

(28% total)

RH

CW

Group 8

(32% total)

RH

CW

Group 9

(36% total)

RH

CW

Group 10

(40% total)

RH

CW

Group 11

(44% total)

RH

CW

Group 12

(48% total)

RH

CW

Group 13

(52% total)

RH

CW

Group 14

(56% total)

RH

CW

Group 15

(60% total)

RH

CW

Group 16

(64% total)

RH

CW

Group 17

(68% total)

RH

CW

Group 18

(72% total)

RH

CW

Group 19

(76% total)

RH

CW

Group 20

(80% total)

RH

CW

Group 21

(84% total)

RH

CW

Group 22

(4%)

RH

CCW

Group 23

(4%)

NRH

CW

Group 24

(4%)

NRH

CCW

Group 25

(4%)

RH=right-handed, NRH=non-right-handed; CW=clockwise hair-whorl rotation, CCW=counterclockwise hair-whorl rotation; BOLD = dominant allele groups 1-21 (84% total), Bold Italics = recessive allele groups 22-25 (4% each).