Genetic and Environmental Diversity in Verbal and Non-verbal Intelligence of Twins from 3-5 years

Annu1 , Bimla Dhanda2*

1Human Development and Family Studies Department, I.C. College of Home Science, Chaudhary Charan Singh Haryana Agricultural University, Hisar-125004, Haryana, India

2I.C. College of Home Science, Chaudhary Charan Singh Haryana Agricultural University, Hisar-125004, Haryana, India

Corresponding Author Email: bimladhanda@gmail.com

DOI : https://doi.org/10.5281/zenodo.7887684

Keywords

Genetic, Non-verbal intelligence, Verbal intelligence

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Abstract

Genes accounted for 50-66% of the verbal and nonverbal intelligence of twins. Intelligence is considered the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experiences. The objective of the present twin study was to assess the diversity of genetic material and environmental factors in the verbal and non-verbal intelligence of twins. A twin study was conducted in two districts, namely, Bhiwani and Hisar of Haryana state. A sample of 100 pairs of twins in the age groups of 3-5 years was attained in villages of Bhiwani district and Hisar district. The intelligence of twins was assessed with help of the Stanford Binet Intelligence Scale [1], whereas the home environment of twins was assessed by using the Home Observation for Measurement of the Environment (HOME) [2]. Results of the present twin study indicated that a highly significant difference in mean values was observed in age of twins for verbal and non-verbal intelligence of twins with the age group 4-5 years of twins of Bhiwani and Hisar district on the basis of the standard test. The findings on heritability estimates showed that the influence of genetics on verbal and non-verbal intelligence level of twins was more than the environment in both districts. The non-verbal intelligence of twins was more influenced by genetic factors as compared to verbal intelligence. The home environment of twins especially learning stimulation in home settings improved the intelligence of twins in early years of life.

Introduction

            The diversity of individual differences in intelligence of twins due to genetic and environmental factors has been one the major concern of behaviour genetics and neuroscience since the very beginning of the field. Intelligence is one of the most reliable psychological trait and characteristics of human behaviour. Individual differences in intelligence remain stable across the life span [3].  [4] revealed that intellectual abilities are one of the strongest predictors of lifelong success, health, longevity, educational outcomes, psychological wellbeing, income, and employment status. Intelligence is a strongly heritable psychological trait, and the integration of neuropsychology and genetics has become increasingly important as rapid advances in gene discoveries [5]. The influence of genetic on intelligence and other psychological traits was assessed by using heritability estimates. Heritability is a statistics that describes the amount of variance in a trait that can be attributed to genetic differences in a given population [6]. It is not a constant value and is varies for a trait depending on developmental processes like neonate, child and adult. Twin study design estimates the relative contribution of heritability in shaping the intelligence of twins [7]. The contribution of genetic predisposition increases in course of the development of intelligence [8].

Both genetic heritage and environmental circumstances are responsible for varies in the intelligence of twins starting from the prenatal period [9].  Genetic studies have shown that intellectual abilities are heritable, and highly polygenic, and that shared genetic factors account for part of their observed co-variation [10]. Genetic influences are an important component of variation for almost all human traits. The research is well agreed that intelligence is one of the most complex behavioural trait, is highly polygenic [9]. The numbers of genes are responsible for affecting the different behavioural traits. [11] stated that specific intellectual abilities are determined by circumscribed sets of function-specific genes but that general intelligence is likely be affected by nearly any gene, anywhere on the genome.   

The genetic material is the strong driving force behind the stability in verbal and non-verbal intelligence of twins. The heritability of verbal and non-verbal intelligence increases with age, ranging from 48 per-cent for verbal intelligence to 64 per-cent for non-verbal intelligence at age 5 [12]. A longitudinal study showed that the stability in verbal and non-verbal intelligence of twins was mainly due to genetic endowment during childhood. The variations in verbal and non-verbal intelligence of twins were accounted by heritability estimates which increase across the lifespan [13].  [14] conducted a longitudinal twin study to assess the genetic and environmental contributions to the temporal stability of verbal, non-verbal and general intelligence across a developmental period spanning childhood and adolescence and revealed that the high stability of additive genetic factors for verbal, non-verbal and general intelligence of twins and stability of environment was moderate. 

Early childhood is considered a valuable time for brain development by many researchers, which comprises cognitive, physical, socio-emotional and language development. Early childhood period is most important for intellectual development, later educational performance as well as for lifelong learning and well-being [15].  Early childhood is as a period in development in which the flow of gene-environment transactions is likely to begin. The differences in early intellectual performance have long- lasting consequences for later intellectual performance [16]. The shared family environment should possibly be the driving force behind an experiential influence on intellectual abilities, for the reason that shared family environmental influences are greatly correlated across different ages and their effects can accumulate across developmental patterns, while individual-specific environmental circumstances are likely to be more age-specific [17].

Materials and Methods

Study Design: The descriptive and experimental research design was used to conduct a twin investigation. The twin research has been planned with the aim to analyze the influence of genetics and environment on verbal and non-verbal intelligence of twins. To conduct this study, mainly two districts were selected randomly namely, Bhiwani and Hisar of Haryana state. To assess the verbal and non-verbal intelligence of twins, total 100 pairs of twins were selected from two districts with the age group 3-to-5 years.

Data collection: For data collection, various methods of data collection were used namely, assessment, interview, observation, and questionnaire methods from the twins and parents of twins to gather relevant information. The snow ball sampling was also used to collect the twins in the required age group of twins.

Tool: Stanford Binet Intelligence Scale [1] was used to assess verbal and non-verbal intelligence of twins. The home environment of twins was measured through Home Observation for Measurement of the Environment (HOME) [2].

Statistical Analysis: The software SPSS (Statistical Package for the Social Sciences) was used to analyze the data and to draw inferences. Mean, Standard Deviation, z-test, Chi-square test, and Heritable estimate were used to meet the objectives of the twin investigation. Heritability estimates (h2) were calculated by the following formula given by [18], h2= 2(RMz- RDz) Where, h2 is the heritability estimate, RMz is the correlation coefficient for monozygotic twin pairs and RDz is the correlation coefficient for dizygotic twins.

Research findings

The results obtained from the present twin investigation have been summarized under the following heads:

Comparison of verbal and non-verbal intelligence of twins from 3-5 years in two districts

As data presented in Table 1 there was a highly significant (0.01%) difference in mean values was observed at age 4-5 years (Z=3.16**) for verbal intellectual twins, but no significant difference in mean values was found on age 3-4 years (Z=1.84) in both Bhiwani and Hisar district, while for non-verbal intelligence, the highly significant in mean values was observed at age 4-5 years (Z=2.58**), but no significant difference was observed for remaining age at 3-4 years (Z=1.60) over the districts. The verbal and non-verbal intelligence of twins was more in Bhiwani district as compared to Hisar district.  

Table 1: Comparison of verbal and non-verbal intelligence level of twins from 3-5 years in two districts

Age (in years)Verbal intelligence
BhiwaniHisarZ value
3-448.87±17.6444.14±18.711.84
4-535.31±10.9239.86±9.383.16**
Non-verbal intelligence
3-420.16±5.0518.95±5.651.60
4-522.03±11.4225.69±8.432.58**

Heritability estimates for verbal and non-verbal intelligence of twins with the age group 3-4 years in both districts

            The data in Table 2 showed that the heritability estimate for verbal intelligence of twins was 54.00 per cent in Bhiwani district and 50.00 per cent in Hisar district in the age group 3-4 years. The interpretation of data clearly revealed that the remaining 45.00 per cent and 50.00 per-cent variance in the intelligence of twins was attributed to environmental factors in Bhiwani and Hisar districts respectively. Further, this table also revealed that the heritability estimates for non-verbal intelligence of twins in the age group 3-4 years was 56.00 per cent in Bhiwani district. The interpretation of data indicated that the remaining 44.00 per-cent variance in the intelligence of twins was due to environmental circumstances in Bhiwani district. In Hisar district, the heritability estimate for the non-verbal intelligence of twins was 52.00 per-cent and the remaining 48.00 per-cent variance in non-verbal intelligence of twins wasattributed to environmental circumstances. The findings on heritability estimates showed that the contribution of genetics was more on the non-verbal intelligence of twins than verbal intelligence in both Bhiwani and Hisar district.                                                        

Table 2:  Heritability estimates for verbal and non-verbal intelligence of twins with the age group 3-4 years in both districts  

DistrictVerbal Intelligence (%)Non-verbal Intelligence (%)
Bhiwani54.0056.00
Hisar50.0052.00

Fig. 1: Heritability estimates for verbal and non-verbal intelligence of twins with the age group 3-4 years in both districts

Heritability estimates for verbal and non-verbal intelligence of twins with the age group 4-5 years in both districts  

As data presented in Table 3 indicated that the heritability estimates for verbal intelligence of twins in the age group 4-5 years were 59.00 per cent in Bhiwani district. The interpretation of data indicated that the remaining 41.00 per-cent variation in the verbal intelligence of twins was due to environmental factors. Further, this table also revealed that the heritability estimate for the verbal intelligence of twins was 54.00 per-cent and the  remaining 46.00 per cent variations in verbal intelligence of twins was attributed to environmental factors in Hisar district. The heritability estimates for non-verbal intelligence was 66.00 per-cent and 60.00 per-cent in Bhiwani and Hisar district respectively. The interpretation of data clearly indicated that the remaining 34.00 per-cent and 40.00 per-cent variations in non-verbal intelligence of twins were due to environmental circumstances in Bhiwani and Hisar district respectively. The findings on heritability estimates revealed that the influence of genetics was more on the non-verbal intelligence of twins as compared to the verbal intelligence of twins in both Bhiwani and Hisar district during 4-5 years.  

Table 3:  Heritability estimates for verbal and non-verbal intelligence of twins with the age group 4-5 years in both districts 

DistrictVerbal (%)Non-verbal (%)
Bhiwani59.0066.00
Hisar54.0060.00

Fig.2: Heritability estimates for verbal and non-verbal intelligence of twins with the

age group 4-5 years in both districts

Correlation co-efficient among monozygotic and dizygotic twins for verbal intelligence in both districts

            The data in Table 4 revealed the correlation coefficient of monozygotic and dizygotic twins for verbal intelligence of twins from 3-4 years. The data clearly indicated that the correlation coefficient(r) of monozygotic twins for verbal intelligence in the age group 3-4 years was 0.36 in Bhiwani district and 0.46 in Hisar district.  Further, the data in this table portrait regarding the dizygotic twins, the correlation coefficient(r) was 0.09 and 0.21 in Bhiwani and Hisar years respectively for the verbal intelligence of twins during 3-4 years. Further, this table also indicated that the correlation coefficient of monozygotic and dizygotic twins for verbal intelligence of twins from 4-5 years, the correlation co-efficient of monozygotic twins was (r=0.89) in Bhiwani district and  (r=0.51) in Hisar district, while for dizygotic twins, correlation co-efficient was 0.59 and 0.24 in Bhiwani and Hisar district respectively. The result provided robust evidence that the monozygotic twins were more correlated with each other than the dizygotic twins for verbal intelligence.

Table 4: Correlation co-efficient among monozygotic and dizygotic twins for verbal intelligence in both districts

DistrictCorrelation co-efficient (r) for verbal intelligence
Monozygotic twinsDizygotic twins
3-4 years
Bhiwani0.360.09
Hisar0.460.21
4-5 years
Bhiwani0.890.59
Hisar0.510.24

Fig.3: Correlation co-efficient among monozygotic and dizygotic twins for verbal intelligence in both districts

Correlation co-efficient among monozygotic and dizygotic twins for non-verbal intelligence in both districts

            The data in Table 5 revealed the correlation coefficient of monozygotic and dizygotic twins for non-verbal intelligence of twins from 3-4 years. The data clearly indicated that the correlation coefficient(r) of monozygotic twins for non-verbal intelligence in age group 3-4 years was 0.44 in Bhiwani district and 0.47 in Hisar district.  Further the data in this table portrait regarding the dizygotic twins, the correlation coefficient(r) was 0.15 and 0.20 in Bhiwani and Hisar years respectively for non- verbal intelligence of twins during 3-4 years. Further, this table also indicated that the correlation coefficient of monozygotic and dizygotic twins for non-verbal intelligence of twins from 4-5 years, the correlation co-efficient of monozygotic twins was (r=0.86) in Bhiwani district and  (r=0.41) in Hisar district, while for dizygotic twins, correlation co-efficient was 0.53 and 0.11 in Bhiwani and Hisar district respectively. The result provided robust evidence that the monozygotic twins were more correlated with each other than the dizygotic twins for non-verbal intelligence.

Table 5: Correlation co-efficient among monozygotic and dizygotic twins for non-verbal intelligence in both districts

DistrictCorrelation co-efficient (r) for non-verbal intelligence
Monozygotic twinsDizygotic twins
3-4 years
Bhiwani0.440.15
Hisar0.470.20
4-5 years
Bhiwani0.860.53
Hisar0.410.11

Fig.4: Correlation co-efficient among monozygotic and dizygotic twins for non-verbal intelligence in both districts

Association of intelligence of twins with home environment in Bhiwani district

The data in Table 6 indicated that the intelligence of twins was associated with dimensions of home environment, namely, learning stimulation (χ2=32.44*), but no association was found between intelligence and one dimension of the home environment, namely, warmth and affection (χ2=1.00) at Bhiwani district.

Table 6: Association of intelligence of twins with home environment in Bhiwani district

Home environmentIntelligence
LowModerateHighTotalχ2
Learning stimulation
Below average55(31.61)57(32.76)18(10.34)130(74.71)32.44*
Above average5(2.87)15(8.62)24(13.79)44(25.29)
Total60(34.48)72(41.38)42(24.14)174(100.00)
Warmth and affection
Below average5(2.87)10(5.75)5(2.87)20(11.49)1.00
Above average55(31.61)62(35.63)37(21.26)154(88.51)
Total60(34.48)72(41.38)42(24.14)174(100.00)

Association of intelligence of twins with home environment in Hisar district

The data in Table 7 revealed that the intelligence of twins was associated with the dimension of home environment, namely, learning stimulation (χ2=8.11*), but no association was found between intelligence and one dimension of home environment, namely, warmth and affection (χ2=3.59) at Hisar district.

Table 7: Association of intelligence of twins with home environment in Hisar district

Home environmentIntelligence
LowModerateHighTotalχ2
Learning stimulation
Below average46(36.51)42(33.33)16(12.70)104(82.54)8.11*
Above average5(3.97)8(6.35)9(7.14)22(17.46)
Total51(40.48)50(39.68)25(19.84)126(100.00)
Warmth and affection
Below average5(3.97)5(3.97)6(4.76)16(12.70)3.59
Above average46(36.51)45(35.71)19(15.08)110(87.30)
Total51(40.48)50(39.68)25(19.84)126(100.00)

Discussion

The major findings of the twin investigation clearly stated that the verbal and non-verbal intelligence of twins were more influenced by genetic material than environmental factors over the districts. The impact was genetics was more observed on the non-verbal intelligence of twins than the verbal intelligence of twins during early childhood. The some similar findings, [19] supported the present twin investigation that the influence of genetics was more likely to the variation in the verbal and non-verbal intelligence of twins than non-shared environmental factors. [20] suggested that the G × E interaction exists for the verbal intelligence of twins in early childhood and tend to be in the direction of greater heritability in high environmental circumstances. Another twin investigation line with the present twin research and concluded that significant genetic and non-shared environmental influences were found for nonverbal intelligence at age 3 years. Moreover, the effect of genetic predisposition was approximately three-quarters of the variance in non-verbal intelligence of twins throughout the early childhood period [21]. Behavioral genetic studies have also the early verbal intelligence aspect such as vocabulary is more influenced by shared environmental circumstances than the genetic factors in early childhood [22].  The high significance of parental and other inputs impact was observed on early verbal intelligence of twins than the non-verbal intelligence of twins in early years of life. [23] assessed  the verbal and nonverbal intelligence of twins by conducting longitudinal twin studies at age 2, 3, and 4 years and reported that 2-year stability coefficients ranging from 0.36 to 0.49 and also suggested that the general cognitive ability can also be measured reliably in early life.

            The results of study revealed that the monozygotic twins were more correlated than dizygotic twins for both verbal and non-verbal intelligence of twins over districts. The results of the study line with [24] examined correlations of intelligence heritability in identical and fraternal twins, and in adopted children as well and observed that the correlation is relatively higher in monozygotic twins than in dizygotic twins. The stability of intelligence and reported that intelligence is the most stable psychological trait, with a Pearson correlation coefficient of 0.54 for monozygotic twins which is more than the dizygotic twins for the intelligence of twins [25].

            The home environment of twins boosted their intelligence of twins. Another twin study provided consistent results,  [26] conducted twin studies have focused on assessing the role of environmental influences on intellectual abilities and applied genetically informative longitudinal methods and by using the monozygotic twin differences design allows for the examination of environmental influences that are unique to each twin, free from the confounds of genetic and shared environmental effects and revealed that monozygotic twins are genetically identical and grow up in the same home environment and may be similar in general intellectual abilities. [27] emphasized that the quality of home environment is considered as a predictor for intelligence, health, education and emotional outcomes. However, the differences in child’s intelligence are related to home environment differences in the presence of maternal characteristics. [28] reported that a significant association between home environment, socioeconomic status, maternal intelligence and early intellectual development in children and also revealed that lower socioeconomic status was associated with smaller volumes of grey matter in several brain areas.

Conclusion

            The present twin research concluded as the diversity of verbal and non-verbal intelligence of twins during early childhood is more influenced by genetic endowment than environmental circumstances.  The genes are more responsible for variations in the non-verbal intelligence of twins than verbal intelligence of twins and environmental factors.  The more similarity in verbal and non-verbal intelligence of twins was in monozygotic twins than dizygotic twins. The learning stimulation at home settings uplifts the intelligence of twins. The home environmental circumstances are significantly associated with the intelligence of twins in the early years of life.

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