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The hereditary basis of mating preference: An investigation carried out on MZ and DZ twins
B. Chiarelli Institute of Anthropology, University of Florence, via del Proconsolo12, Florence, Italy
E. Rabino Massa Department of Animal Biology, University of Turin, via Accademia Albertina 17, Turin, Italy Keywords: sexual preferences in
man, twins, anthroposcopic traits.
The Hereditary Basis of Mating Preference, An Investigation Carried Out on MZ and DZ Twins The existence of a genetic basis in the choice of a sexual partner was examined by comparing the physiognomic preferences of 62 twin pairs (31 MZ, 31 DZ). The twin pairs were questioned separately on their preference of 13 physiognomic traits. An analysis of their responses was undertaken to determine whether MZ twins were more concordant than DZ in their preferences. By discussing simple and crossed concordances, there was 68"6% concordance for MZ twins and 57-5% for DZ twins (significant at P<0-01). This lends support to the hypothesis that the preference for some physiognomic traits have a hereditary basis.
Journal of Human Evolution (1985) 14, 379-384
1. I n t r o d u c t i o n
All cultures have their own standards of beauty of the h u m a n body, both for males and for females. Beauty contests are actually a formalization of such standards. Another question is to what extent these standards are determined by cultural versus genetic factors. Even though Darwin (1871) negated the existence of a universal standard of beauty, he believed that some preferences might become standards of beauty in certain populations. This should lead to the establishment of specific physical traits. Undoubtedly, some traits function as sexual attractants both in m a n and in other species (Tinbergen, 1960; Smith, 1967; Tinbergen & Falcus, 1970; Wilson, t972). T h e inherent genetic c o m p o n e n t of a stimulus system is suggested by the consistent responses which such stimuli elicit from previously unexposed y o u n g individuals, a n d / o r because such stimuli are recognized and used by several animal species. H u m a n s also recognize a n u m b e r of physical characteristics as "stimulus signals". Cultural factors, however, m a y alter their value or m a y add new signals of an exclusively social or cultural nature (Mainardi, 1968; Gallup & Suarez, 1983; Chiarelli, 1984). T h e function of stimulus signals is not restricted to primary sexual features. These signals are associated with a complex of physical traits which collectively exert some influence on the preferential choice of a partner. Indeed, the different appreciation of different individuals for a specific ensemble of such characteristics points to the existence of a possible genetic substrate which m a y determine their choice, and hence the object of their sexual selection. M o n t a g u has already emphasized such a possibility stating that "functionally sexual selection could be defined as the process of selecting mates on the basis of a preferred s t a n d a r d of beauty or other desirable quality, so that in the course of time the sexually preferred type would become the d o m i n a n t one in the group, and perhaps cause the non-preferred type to become a separate isolate, or even to die out" (1960, p. 402). A cumulative list of husband/wife correlations in physical characteristics in m a n has been recently presented by Roberts (1977). T h e purpose of the present investigation is to reveal the possible involvement of genetic factors in the choice of a partner. More precisely, an attempt is made to develop a suitable 0047-2484/85/040379 + 06 $03.00/0
~) 1985 Academic Press Inc. (London) Limited
380
B. CHIAREI~LIAND E. RABINOMASSA
method for demonstrating the existence, if any, of a genetic basis underlying the preference for some physiognomic characteristics.
2. Material and Methods According to a formerly adopted scheme (Conterio & Chiarelli, 1962a), 72 twin pairs of the same sex between 17 and 21 years of age (i.e., 39 female pairs and 33 male pairs) were studied. They were given a questionnaire to record their physiognomic preferences (see below). An attempt was made to determine whether MZ twins concord more than DZ twins in their choices: such a higher concordance might indicate the influence of hereditary factors in this respect. O f the 72 pairs who volunteered, the zygosity was determined on the basis of blood groups, anthropometric characters and dermatoglyphics. Thirty-one pairs were MZ and 31 were DZ, while the remaining 10 pairs were not diagnosable as to their zygosity and were discarded (Alciati, 1960; Brismar, 1968; Conterio & Chiarelli, 1962b; Nylander, 1971; Parisi & Di Bacco, 1968; Weninger et al., 1976). The following anthroposcopic traits were selected. (1) Nose shape. Martin & Saller's (1957, p. 407) table was used. This table provides a number of type variants classified into the following three categories: upturned nose, straight nose, hooked or crooked nose. Fifteen nose shapes were used. (2) Mouth shape. Again, Martin & Saller's (1957, p. 409) table was followed because it encompasses the range of variability of Caucasoid populations, even taking into account Comas' (1960) and Beals & Hoijer's (1959) observations. A classification into six mouth shapes was used. (3) Shape of the epicanthic fold. Concerning this trait a modified version of Weiner & Lourie's (1969, p. 198) table was used, in which different forms of the upper eyelid were subdivided by these workers into five male and five female types. In our own questionnaire both complete and incomplete shapes of the eyelid were included, because they also occur in our populations, although with a low frequency (Topinard, 1885; Biasutti, 1953). (4) Size and slant of the eye. Rostand & Tfitry's (1965) schemes were followed. Eyes were called "wide" when the sclera was visible at least as a narrow strip between the lower margin of the iris and the eyelid. We classified as "slanted" those eyes in which the lateral angles were set over 6 mm higher than the medial ones, because the normal angle divergence in height lies between 4 and 6 mm (Chiarugi, 1926). Two categories for each sex were used. (5) Forehead shape. The forehead measurements and scheme (in Martin &Saller, 1957, p. 406; according to Weninger) were used. Four variants of the glabellar prominence were considered. (6) Hairshape. Martin & Saller's (1957, p. 393) table was used, from which only the two pictures of Bushman and Hottentot hair shapes were eliminated. (7) Handshape. Kfihnel's scheme in Martin & Saller (1966, p. 2462) was resorted to. On the basis of these schemes (without changing the proportions) pictures were constructed which were more realistic and easier to interpret. Six variants of hand types were used. (8) Finger shape. We used Koenner's table reported by Martin &Saller (1957, p. 425), in which 11 phenotypic forms of the last phalanx and nail are represented. (9) Face shape. Poch's table, reported by Martin & Sailer (1957, p. 403), was used with slight modifications for the female sex.
381
THE HEREDITARY BASIS OF MATING PREFERENCE
Table 1
Type of choice examined 1
2
3
Choice 1st 2nd
Choice 1st 2nd
Choice 1st 2nd
Twin A
8
2
4
Twin B
~
3
5
t
2
-..
4
1
4
1 = one simple concordance. 2 --- one crossed concordance. 3 = two crossed concordances.
(10) Iris colour. We did not use any table or set of samples. The iris was classified as dark (when brown or black) or light (when light blue or green). (11) Body shape. A table concerning the somatic types was included in the questionnaire. Following the typology by Sheldon (1954) we set up our table using some of the examples collected by this author in his atlas, as well as some of his drawings reported by Martin & Saller (1966, pp. 2468, 2470). (12) Male and female respective heights. Finally, in the questionnaire we inquired about the stature, this trait being specially involved with regard to sexual choice, and hence worthy of our attention. (13) Hair colour. In the questionnaire only one question was asked about the favourite hair colour, without referring to any table or set of samples. This question was excluded from the evaluation of concordances. Trait selection and tables took into account further works on anthropology and anthropometry, among them Olivier's (1960, 1961) investigations, as well as general studies on twins, e.g., Loehlin et al. (1976) and Gedda (1951). When making the layout of the present anthroposcopic tables, care was taken to introduce as few variables as possible, in order to obtain the clearest possible insight into Table 2
Percentage ratios for concordances of monozygotic (MZ) and dizygotic (DZ) twins
Question number
Simple concordances MZ DZ
Crossed concordances MZ DZ
Simple + crossed concordances MZ DZ
l 2 3 4 5 6 7 8 9 10 11 12
46.6 37"7 43'5 48.3 50"2 55'7 52.4 34"7 41.1 48.7 56"3 49'4
35'4 38"2 44'1 46.3 40.1 56-3 55"5 37-6 33'2 50.1 58"1 48'5
14.5 6"7 22"3 18.2 21.7 10-3 24-1 18.1 26.1 7'9 28"6 17-7 21'9 14.7 12"9 8"2 t6'9 6.8 22.1 12.0 24'9 16'3 23"5 12.0
61.1 60.0 65"2 72"4 76.3 84.3 74"3 47.6 58'0 70.8 81"2 72"9
42"1 54.6 54.4 64.4 48.0 74-0 70-2 45'8 40.1 62.1 74.4 60,5
Mean
47'0
45'3
21"5
68"6
57.5
12.4
382
B, C H I A R E L L I A N D E. R A B I N O M A S S A
Figure 1. Diagram representing M Z a n d D Z preferences. 8C
7C
hAh
o b +
Meon 6 8 6 %
/
F] 60
Mean_57-5~
E
. . . .
cn
50 E
8
40
"5
8, o
30
Y
20
t I
10
4 '5 6 7 8 9101112 MZ
Ik III 111
2 :5 4 5 6 7 B 910"H~2 OZ
the preferences under study. Photographic material was discarded, because it may introduce other variables which are difficult to evaluate. For some physical traits it was necessary to prepare a male and a female version (e.g. shape of the epicanthic fold, size and slant of the eye, face shape and body shape). The members bf each twin pair were questioned separately. For each trait two choices were allowed, a first and a second choice. In addition to the questions pertaining to the sexual choice, preferences relating to colour, sports, films and other entertainments were included in the questionnaire. In this way the information on the degree of taste affinity between twin-members was extended to everyday life, thus providing the possibility of comparison with the physiognomic preferences. 3. Results and D i s c u s s i o n An analysis of the results was undertaken in order to evaluate whether MZ are more concordant than DZ twins in their preferences towards the somatic traits under examination. We recorded first of all the number of concordances for each pair, i.e., the number of similar responses about the sexual choice according to the questionnaire. Because two choices were allowed for each trait, two possibilities may arise: (1) the first choice by one twin-member may coincide with the first choice by the other (or, the second choice by one member with the second by the other);
T H E H E R E D I T A R Y BASIS OF MATING P R E F E R E N C E
383
(2) the first choice by one twin-member may coincide with the second choice by the other. For simplicity's sake the first possibility will be referred to as simple concordance, the second as crossed concordance. Examples are given in Table 1. T h e average numbers of concordances in M Z (31 twin pairs) and D Z (30 pairs) were calculated. O n e D Z male pair whose questionnaire was not completely filled out was excluded (Table 2 and Figure 1). By adding simple and crossed concordances we obtained a 68"6% concordance for M Z twins and 57"5% for D Z twins. Analysis by the Student t-test revealed that the difference between these values is significant (P<0"01). This lends strong support to the hypothesis that the preference for the physiognomic traits under consideration have a hereditary basis. W h e n only the simple concordances are evaluated we obtain percentages of 47'0% and 45"3% for M Z and DZ twins, respectively. This difference is not significant. T h e order of preference is apparently strongly affected by the environment (e.g., visual stimuli due to advertisements, television or films) or by the contingent psychic state of the individual. T h e type of preference seems to be more stable. As to the concordance in the replies to other questions (i.e. preferences for certain colours, films, sports and other entertainments) we scored a mean value of 8"033 for M Z twins and 7"323 for DZ twins. This difference is not significant (Student t-test). T h e correlation between the concordances in sexual choice and in the other investigated fields was examined by the m e t h o d of rank correlation. T h e correlation coefficient obtained (0.064) shows that there is virtually no correlation between sex preferences and the other preferences considered here; hence, those individuals who prefer the same somatic features m a y have different tastes in other fields, and vice versa. T h e low rank correlation coefficient was confirmed by the B r a v a i s - P e a r s o n coefficient (0'07). I n view of the above it seems reasonable to state that a genetic basis underlies the choice of a partner, at least in as far as a general tendency towards some physical types is concerned. All the findings reported here seem to substantiate this assumption. It is hoped that the availability of larger samples of twins may lead to the confirmation of the foregoing preliminary results.
References
Alciati, G. (1960). SuUa metodologia per 1o studio dei rilievi cutanei palmari. Rivista di Antropologia 47, 88-127. Beals, R. L. & Hoijer, H. (1959). An Introduction to Anthropology. New York: Macmillan. Biasutti, R. (1953). Le razze e i popoli della terra, Vol. 1. Unione tipografico-editrice torinese. Brismar, B. (1968). Dermatoglyphics of twins. Acta Geneticae Medicae et Gemellologiae 17, 375-380. Chiarugi, G. (1926). Istituzioni di anatomia sistematica. 2nd Edn, Vol. 1, Milan: Societh Editrice Libraria. Chiarelli, B. (1984). Origine della socialita' e della cultura umana. Bark Laterza. Comas, J. (1960). Manual of Physical Anthropology. Springfield, IL: Charles C. Thomas. Conterio, F. & Chiarelli, B. (1962a). Studio sulla ereditabilita' di alcuni caratteri antropometrici misurati sui gemelli. L'Ateneo parmense 33, 1-7. Conterio, F. & Chiarelli, B. (1962b). Study of the inheritance of some daily life habits. Heredity 17, 347-359. Darwin, C. (1871). The Descent of Man and Selection in Relation to Sex, Vols 1 and 2. London: John Murray. Gallup, G. G. & Suarez, S. D. (1983). Optimal reproductive strategies for bipedalism. Journal of Human Evolution 12, 193-196. Gedda, L. (1951). Studio dei gemelli. Rnma: Edizioni Orizzonte Medico. Loehlin, J. C. & Nichols, C. (1976). Heredity, Environment and Personality: a Study of 850 Sets of Twins. Austin and London: University of Texas.
384
B. CHIARELLI AND E. RABINO MASSA
Mainardi, D. E. (1968). La scelta sessuale, Turin: Boringhieri. Martin, R. & Sailer, K. (1957). Lehrbuch der Anthropologie in Systematischer Darstellung, Vol. 1. Stuttgart: Gustav Fischer Verlag. Martin, R. & Sailer, K. (t966). Lehrbuch der Anthropologie in Systematischer DarsteUung. VoI. 4. Stuttgart: Gustav Fischer Verlag. Montagu, M. F. (1960). An Introduction to Physical Anthropology. 3rd Edn. Springfield, IL: Charles C. Thomas. Nylander, P. P. (1971). Fingerprints and the determination of zygosity in twins. American Journal of Physical Anthropology 35, 101-108. Olivier, G. (1960). Pratique Anthropologique. Paris: Vigor Fr~res. Olivier, G. (1961). Morphologie et types humains. Paris: Vigot Fr~res. Parisi, P. & Di Bacco, M. (1968). Fingerprints and the diagnosis ofzygosity in twins. Acta Geneticae Medicae et Gemellologiae 17, 333-358. Roberts, D. F. (1977). Assortative Mating in Man: Husband/Wife Correlations in Physical Characters. Eugenics Society, supp. 2, September. Rostand, J. & T~try, A. (1965). An Atlas of Human Genetics. London: Hutchinson. Sheldon, W. H. (1954). Atlas of Man. New York: Harper Brothers. Smith, N. G. (1967). Visual isolation in gulls. Scientific American 217, 94-102. Tinbergen, N. (1960). The evolution &behavior in gulls. Scientific American 203, 118-130. Tinbergen, N. & Falkus, H. (1970). Segnali per sopravvivere. Italian translation 1976. Bologna: Zanichelli. Topinard, P. (1885). Elements d'anthropologie ggngrale, pp. 998-1003. Paris: Adrien Delahaye et Emile Lecrosnier. Weiner, J. S. & Lourie, J. A. (1969). Human Biology: a Guide to Field Methods. Oxford and Edinburgh: Blackwell Scientific Publications. Weninger, M., Aue-Hauser, G. & Scheiber, V. (1976). Total finger ridge-count and the polygenic hypothesis: a critique. Human Biology 48, 713-725. Wilson, E, O. (1972). Animal communication. Scientific American 227, 52-60.
E. Rabino Massa Department of Animal Biology, University of Turin, via Accademia Albertina 17, Turin, Italy Keywords: sexual preferences in
man, twins, anthroposcopic traits.
The Hereditary Basis of Mating Preference, An Investigation Carried Out on MZ and DZ Twins The existence of a genetic basis in the choice of a sexual partner was examined by comparing the physiognomic preferences of 62 twin pairs (31 MZ, 31 DZ). The twin pairs were questioned separately on their preference of 13 physiognomic traits. An analysis of their responses was undertaken to determine whether MZ twins were more concordant than DZ in their preferences. By discussing simple and crossed concordances, there was 68"6% concordance for MZ twins and 57-5% for DZ twins (significant at P<0-01). This lends support to the hypothesis that the preference for some physiognomic traits have a hereditary basis.
Journal of Human Evolution (1985) 14, 379-384
1. I n t r o d u c t i o n
All cultures have their own standards of beauty of the h u m a n body, both for males and for females. Beauty contests are actually a formalization of such standards. Another question is to what extent these standards are determined by cultural versus genetic factors. Even though Darwin (1871) negated the existence of a universal standard of beauty, he believed that some preferences might become standards of beauty in certain populations. This should lead to the establishment of specific physical traits. Undoubtedly, some traits function as sexual attractants both in m a n and in other species (Tinbergen, 1960; Smith, 1967; Tinbergen & Falcus, 1970; Wilson, t972). T h e inherent genetic c o m p o n e n t of a stimulus system is suggested by the consistent responses which such stimuli elicit from previously unexposed y o u n g individuals, a n d / o r because such stimuli are recognized and used by several animal species. H u m a n s also recognize a n u m b e r of physical characteristics as "stimulus signals". Cultural factors, however, m a y alter their value or m a y add new signals of an exclusively social or cultural nature (Mainardi, 1968; Gallup & Suarez, 1983; Chiarelli, 1984). T h e function of stimulus signals is not restricted to primary sexual features. These signals are associated with a complex of physical traits which collectively exert some influence on the preferential choice of a partner. Indeed, the different appreciation of different individuals for a specific ensemble of such characteristics points to the existence of a possible genetic substrate which m a y determine their choice, and hence the object of their sexual selection. M o n t a g u has already emphasized such a possibility stating that "functionally sexual selection could be defined as the process of selecting mates on the basis of a preferred s t a n d a r d of beauty or other desirable quality, so that in the course of time the sexually preferred type would become the d o m i n a n t one in the group, and perhaps cause the non-preferred type to become a separate isolate, or even to die out" (1960, p. 402). A cumulative list of husband/wife correlations in physical characteristics in m a n has been recently presented by Roberts (1977). T h e purpose of the present investigation is to reveal the possible involvement of genetic factors in the choice of a partner. More precisely, an attempt is made to develop a suitable 0047-2484/85/040379 + 06 $03.00/0
~) 1985 Academic Press Inc. (London) Limited
380
B. CHIAREI~LIAND E. RABINOMASSA
method for demonstrating the existence, if any, of a genetic basis underlying the preference for some physiognomic characteristics.
2. Material and Methods According to a formerly adopted scheme (Conterio & Chiarelli, 1962a), 72 twin pairs of the same sex between 17 and 21 years of age (i.e., 39 female pairs and 33 male pairs) were studied. They were given a questionnaire to record their physiognomic preferences (see below). An attempt was made to determine whether MZ twins concord more than DZ twins in their choices: such a higher concordance might indicate the influence of hereditary factors in this respect. O f the 72 pairs who volunteered, the zygosity was determined on the basis of blood groups, anthropometric characters and dermatoglyphics. Thirty-one pairs were MZ and 31 were DZ, while the remaining 10 pairs were not diagnosable as to their zygosity and were discarded (Alciati, 1960; Brismar, 1968; Conterio & Chiarelli, 1962b; Nylander, 1971; Parisi & Di Bacco, 1968; Weninger et al., 1976). The following anthroposcopic traits were selected. (1) Nose shape. Martin & Saller's (1957, p. 407) table was used. This table provides a number of type variants classified into the following three categories: upturned nose, straight nose, hooked or crooked nose. Fifteen nose shapes were used. (2) Mouth shape. Again, Martin & Saller's (1957, p. 409) table was followed because it encompasses the range of variability of Caucasoid populations, even taking into account Comas' (1960) and Beals & Hoijer's (1959) observations. A classification into six mouth shapes was used. (3) Shape of the epicanthic fold. Concerning this trait a modified version of Weiner & Lourie's (1969, p. 198) table was used, in which different forms of the upper eyelid were subdivided by these workers into five male and five female types. In our own questionnaire both complete and incomplete shapes of the eyelid were included, because they also occur in our populations, although with a low frequency (Topinard, 1885; Biasutti, 1953). (4) Size and slant of the eye. Rostand & Tfitry's (1965) schemes were followed. Eyes were called "wide" when the sclera was visible at least as a narrow strip between the lower margin of the iris and the eyelid. We classified as "slanted" those eyes in which the lateral angles were set over 6 mm higher than the medial ones, because the normal angle divergence in height lies between 4 and 6 mm (Chiarugi, 1926). Two categories for each sex were used. (5) Forehead shape. The forehead measurements and scheme (in Martin &Saller, 1957, p. 406; according to Weninger) were used. Four variants of the glabellar prominence were considered. (6) Hairshape. Martin & Saller's (1957, p. 393) table was used, from which only the two pictures of Bushman and Hottentot hair shapes were eliminated. (7) Handshape. Kfihnel's scheme in Martin & Saller (1966, p. 2462) was resorted to. On the basis of these schemes (without changing the proportions) pictures were constructed which were more realistic and easier to interpret. Six variants of hand types were used. (8) Finger shape. We used Koenner's table reported by Martin &Saller (1957, p. 425), in which 11 phenotypic forms of the last phalanx and nail are represented. (9) Face shape. Poch's table, reported by Martin & Sailer (1957, p. 403), was used with slight modifications for the female sex.
381
THE HEREDITARY BASIS OF MATING PREFERENCE
Table 1
Type of choice examined 1
2
3
Choice 1st 2nd
Choice 1st 2nd
Choice 1st 2nd
Twin A
8
2
4
Twin B
~
3
5
t
2
-..
4
1
4
1 = one simple concordance. 2 --- one crossed concordance. 3 = two crossed concordances.
(10) Iris colour. We did not use any table or set of samples. The iris was classified as dark (when brown or black) or light (when light blue or green). (11) Body shape. A table concerning the somatic types was included in the questionnaire. Following the typology by Sheldon (1954) we set up our table using some of the examples collected by this author in his atlas, as well as some of his drawings reported by Martin & Saller (1966, pp. 2468, 2470). (12) Male and female respective heights. Finally, in the questionnaire we inquired about the stature, this trait being specially involved with regard to sexual choice, and hence worthy of our attention. (13) Hair colour. In the questionnaire only one question was asked about the favourite hair colour, without referring to any table or set of samples. This question was excluded from the evaluation of concordances. Trait selection and tables took into account further works on anthropology and anthropometry, among them Olivier's (1960, 1961) investigations, as well as general studies on twins, e.g., Loehlin et al. (1976) and Gedda (1951). When making the layout of the present anthroposcopic tables, care was taken to introduce as few variables as possible, in order to obtain the clearest possible insight into Table 2
Percentage ratios for concordances of monozygotic (MZ) and dizygotic (DZ) twins
Question number
Simple concordances MZ DZ
Crossed concordances MZ DZ
Simple + crossed concordances MZ DZ
l 2 3 4 5 6 7 8 9 10 11 12
46.6 37"7 43'5 48.3 50"2 55'7 52.4 34"7 41.1 48.7 56"3 49'4
35'4 38"2 44'1 46.3 40.1 56-3 55"5 37-6 33'2 50.1 58"1 48'5
14.5 6"7 22"3 18.2 21.7 10-3 24-1 18.1 26.1 7'9 28"6 17-7 21'9 14.7 12"9 8"2 t6'9 6.8 22.1 12.0 24'9 16'3 23"5 12.0
61.1 60.0 65"2 72"4 76.3 84.3 74"3 47.6 58'0 70.8 81"2 72"9
42"1 54.6 54.4 64.4 48.0 74-0 70-2 45'8 40.1 62.1 74.4 60,5
Mean
47'0
45'3
21"5
68"6
57.5
12.4
382
B, C H I A R E L L I A N D E. R A B I N O M A S S A
Figure 1. Diagram representing M Z a n d D Z preferences. 8C
7C
hAh
o b +
Meon 6 8 6 %
/
F] 60
Mean_57-5~
E
. . . .
cn
50 E
8
40
"5
8, o
30
Y
20
t I
10
4 '5 6 7 8 9101112 MZ
Ik III 111
2 :5 4 5 6 7 B 910"H~2 OZ
the preferences under study. Photographic material was discarded, because it may introduce other variables which are difficult to evaluate. For some physical traits it was necessary to prepare a male and a female version (e.g. shape of the epicanthic fold, size and slant of the eye, face shape and body shape). The members bf each twin pair were questioned separately. For each trait two choices were allowed, a first and a second choice. In addition to the questions pertaining to the sexual choice, preferences relating to colour, sports, films and other entertainments were included in the questionnaire. In this way the information on the degree of taste affinity between twin-members was extended to everyday life, thus providing the possibility of comparison with the physiognomic preferences. 3. Results and D i s c u s s i o n An analysis of the results was undertaken in order to evaluate whether MZ are more concordant than DZ twins in their preferences towards the somatic traits under examination. We recorded first of all the number of concordances for each pair, i.e., the number of similar responses about the sexual choice according to the questionnaire. Because two choices were allowed for each trait, two possibilities may arise: (1) the first choice by one twin-member may coincide with the first choice by the other (or, the second choice by one member with the second by the other);
T H E H E R E D I T A R Y BASIS OF MATING P R E F E R E N C E
383
(2) the first choice by one twin-member may coincide with the second choice by the other. For simplicity's sake the first possibility will be referred to as simple concordance, the second as crossed concordance. Examples are given in Table 1. T h e average numbers of concordances in M Z (31 twin pairs) and D Z (30 pairs) were calculated. O n e D Z male pair whose questionnaire was not completely filled out was excluded (Table 2 and Figure 1). By adding simple and crossed concordances we obtained a 68"6% concordance for M Z twins and 57"5% for D Z twins. Analysis by the Student t-test revealed that the difference between these values is significant (P<0"01). This lends strong support to the hypothesis that the preference for the physiognomic traits under consideration have a hereditary basis. W h e n only the simple concordances are evaluated we obtain percentages of 47'0% and 45"3% for M Z and DZ twins, respectively. This difference is not significant. T h e order of preference is apparently strongly affected by the environment (e.g., visual stimuli due to advertisements, television or films) or by the contingent psychic state of the individual. T h e type of preference seems to be more stable. As to the concordance in the replies to other questions (i.e. preferences for certain colours, films, sports and other entertainments) we scored a mean value of 8"033 for M Z twins and 7"323 for DZ twins. This difference is not significant (Student t-test). T h e correlation between the concordances in sexual choice and in the other investigated fields was examined by the m e t h o d of rank correlation. T h e correlation coefficient obtained (0.064) shows that there is virtually no correlation between sex preferences and the other preferences considered here; hence, those individuals who prefer the same somatic features m a y have different tastes in other fields, and vice versa. T h e low rank correlation coefficient was confirmed by the B r a v a i s - P e a r s o n coefficient (0'07). I n view of the above it seems reasonable to state that a genetic basis underlies the choice of a partner, at least in as far as a general tendency towards some physical types is concerned. All the findings reported here seem to substantiate this assumption. It is hoped that the availability of larger samples of twins may lead to the confirmation of the foregoing preliminary results.
References
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