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Twin similarities in Holland types as shown by scores on the self-directed search.

This study examined the degree of similarity between scores on the Self-Directed Search from one set of identical twins. Predictably, a high congruence score was found. Results from a biographical sheet are discussed as well as implications of the results for career counselors.


From the beginning, Holland (1966) described his work as a "heuristic theory ... that stimulates research and investigation by its suggestive character" (p. 8). His research suggestions exhort the reader to investigate interactions such as student-teacher, client-counselor, employee-supervisor, and husband-wife using his typology (Bruch, 1978; Bruch & Skovholt, 1985; Cox & Thoreson, 1977; Helwig & Myrin, 1997; Miller, Springer, & Cowger, 2004; Turner & Horn, 1977; Whitney & Whittlesey, 1972). Thus, extending Holland's (1985) typology to include a relatively uncommon group-identical twins-appears consistent with his heuristic intentions.

Specifically, this study uses a case study to examine the congruency of Holland types in a unique (and rare) subgroup within a family structure: identical twins. Indeed, twin studies have intrigued researchers for decades. Twin studies have been used to assess the influence of genetics in a host of areas, including various personality traits and intelligence (see Myers, 2004). The authors could not locate any study that investigated the degree of similarity between the three-letter Holland codes of identical twins.

After presenting a somewhat detailed overview of Holland's theory, a brief review of the literature using identical twins as subjects is presented as well as a justification for using the case study design.

Holland's (1985, 1997) Theory

The theory of careers proposed by Holland (1985) characterizes persons and environments as a single set of six types. Most people in our culture can be classified as one of six dominant types (Realistic, Investigative, Artistic, Social, Enterprising, Conventional). Thus, an individual's personality (i.e., interest, values, traits, abilities, and fantasies) can be assessed by considering his or her three most dominant types. People are motivated to seek out work environments that complement their dominate personality types, thereby maximizing the chances of individual satisfaction and well-being. Holland (1997) advocated the use of three-letter codes to identify occupations consistent with an individual's personality type.

In Holland's typology, a personality is expressed in a three-letter code. A three-letter code is formed by selecting from Holland's six types, the three types that most closely characterize the person. The three-letter code provides a brief summary of what a person is like. For example, a three-letter code of CER suggests that the person has a dominant Conventional personality, but also possesses the Enterprising and Realistic characteristics to a somewhat lesser degree. Extensive descriptions of the six types can be found in Holland (1985, 1997). A vast number of career instruments are currently available to assess a client's three-letter code, most notably the various versions of the Self-Directed Search (SDS; Holland, 1994).

Holland (1985, 1997) introduced a number of important career-related concepts to the vocational psychology literature. One such concept is congruency. Congruency is defined as the relative proximity in the hexagon between any pair of Holland three-letter codes (Gati, 1985). For this study, the pair of Holland three-letter codes will be determined from the results of the Self-Directed Search by each twin.

Brief Review of Twin Studies

Betsworth et al. (1994) used, as part of their research, twins reared apart and together and found that one-third of the variance associated with vocational interests was genetically influenced. Moloney, Bouchard, and Segal (1991) used twins to examine the genetic and environmental influences and vocational interests. They concluded using the model fitting analysis, that 45-50% of the interest variation, across a wide sample of vocational interests, was genetically influenced. Finally, Nichols (1978) performed a meta-analysis of the twin students of vocational interests. Results revealed that the average of the intraclass correlations across six major interests factors (factors similar to Holland's (1985) RIASEC types-practical, science, business, clerical, helping, and artistic) was .48 for identical twins (monozygotic) reared together and .30 for fraternal twins (dizygotic). Finally, some studies have investigated twins reared together and reported on the influence of genetics (Carter, 1932; Roberts & Johansson, 1974; Vandenburg & Stafford, 1967).

Single Case Study Design

Case study research refers to the detailed examination of a single entity (e.g., client, person, a counseling dyad, a group; Miller, 1985). Single case design was specifically developed for use in applied settings (Bloom, Fischer, & Orne, 1995) and for behaviors such as overt speech, scores on self-report instruments (i.e., SDS), and daily self-ratings. Several researchers have discussed the virtues of single case study research (Frey, 1978; Goldman, 1977; Remer, 1981; Tracey, 1983). According to Hill, Carter, and O'Farrell (1983), single subject research has distinct advantages over more traditional experimental studies because it allows for a more adequate description of what is happening. Sue (1978) adds that single subject research allows one to study a somewhat rare behavior. For this study, we investigated the degree of congruency between one set of identical twins.



Each twin volunteered to participate and received a small honorarium for their involvement. The biographical data sheet and the SDS were administered in the same order to control for possible order effects. The primary author discussed the procedures and answered any questions, guaranteeing privacy and confidentiality of the results.

To assist in locating identical twins on campus, the authors solicited the services of the Vice-President of Student Affairs who was able to locate one set of identical twins. They both volunteered to participate. The twins were female, 19 years of age, white, and single.


Biographical Data Sheet (BDS). A demographic information form designed for this study was used to obtain specific demographic characteristics consistent with the unique nature of this study. The biographical data sheet requested the following personal information: gender, age, race/ethnicity, marital status, academic major, occupational fantasy, leisure pursuits, favorite music, favorite sport, and favorite food.

The Self-Directed Search (SDS; Holland, 1994) is a self-administered, self-scored, and self-interpreted instrument designed to provide career guidance for students and adults by assessing a client's similarity to six personality types. The SDS consists of a number of subtests, and individuals progress through each subtest by responding "Like" or "Dislike" or "Yes" or "No" to most of the items. Self-estimates require individuals to rank themselves on a 1-7 scale on each of six traits related to the six Holland types (RIASEC). After completing the SDS, individuals sum their positive responses and arrive at a three-letter code on the summary page.

Holland (1985) reported KR-20 internal consistency estimated for SDS summary scales ranging from .86 to .91 and 1 to 4 week retest reliability estimates ranging for .70 to .89. Evidence of concurrent validity is indicated by the percentage of agreement between SDS high-point summary codes and the first-letter code of a current vocational aspiration. Holland (1985) reported agreement percentages ranging from 58% to 14%. Several hundred studies have been conducted on the SDS with generally favorable results (Osipow, 1993).

Analysis: Measuring Congruence using the Iachan Index

The concept of congruency continues to be studied widely in the literature (Meir, 1989). For example, an entire issue of the Journal of Vocational Behavior (Spokane, 1985) was devoted to the conceptual and methodological issues of research on congruence. Generally, Holland's concept of congruency has been critiqued favorably a number of times over the last 25 years (e.g., D. Brown, 1987; Carkhuff, Alexik, & Anderson, 1967; Osipow, 1983; Spokane, 1985).

Congruence models have a long history in the field of vocational psychology (Assouline & Meir, 1987; Spokane, 1985). The difficulty of quantifying the similarity between two Holland codes, however, is at the heart of the issue of measuring congruence. Much of the research in vocation psychology concentrates on attempts to clarify this difficulty (Osipow, 1983). In his review, Spokane (1985) listed 8 indices of congruence. Later, Camp and Chartrand (1992) compared 13 different measures that had been developed to operationalize Holland's congruence construct and found meaningful variability in their intercorrelations. The most recent measure of congruence-the Brown and Gore "C" (for congruency)--was introduced in 1994 (S. D. Brown & Gore, 1994).

Because Camp and Chartrand (1992) recommended more sophisticated distance measures of congruence, which would more completely include Holland's hexagon and order assumptions, the Iachan Index (Iachan, 1984) was chosen for this study. Specifically, the Iachan Index (Iachan, 1984) was used because it quantitatively describes the degree of congruency between any two separate three-letter codes and because Holland (1985) recommended it as one of the best measures of congruency.

The Iachan Index (Iachan, 1984) has an elaborate mathematical development, but its use requires only the ability to add to 28. Certain code pairings are given weights. The value for an exact match (i.e., the first letter for both codes are identical) is 22. The value for a close match (i.e., the first letter of one code appears as the second letter of the other code) is 10. The value of a marginal match (i.e., the first letter of one code appears as the third letter of the other code) is 4. If no letters match between codes, the total is 0 (see Iachan, 1984). In theory, higher scores on the Iachan Index indicate greater congruency. Finally, according to Iachan (1984), interpretation of the congruence scores is as follows: (a) scores of 26 to 28 are very close matches, (b) scores of 20 to 25 are close matches, (c) scores 14 to 19 are not close matches, and (d) scores of 13 and below are poor matches. It was speculated that an extremely high degree of congruence (using the Iachan Index, 1984) between the three-letter Holland codes from each twin would suggest the important role played by both genetics and environment in ultimately determining vocational interests.

Results from the SDS

The three letter code for each twin was AI S/E and AR S/E, repectively. The linear additive nature of the Iachan Index makes the treatment of ties (i.e., S/E) very simple. Whenever two or more codes are tied, the index is computed as the average of the individual index values. In the simplest case when only one code is tied, only two index values need to be averaged (Iachan, 1990). Since both three-letter codes in our study had a tied in the third position, a total of four codes were calculated and averaged. Hence, an analysis of the four codes revealed an index score of 22.5. Given the range on the Iachan Index (0-28, higher scores indicate greater congruency), this index score suggests an extreme high degree of congruence between the two scores.

Results from the Biographical Sheet

Obviously, our identical twins were the same age, same sex and same race; and both were single. Their academic majors were undecided and pre-veteranian. Their occupational fantasies and leisure pursuits were compared using an extension of the Iachan congruence index (1990). This 1990 version of the Iachan Index quantitatively describes the degree of congruency using only a two-letter (instead of a three letter) Holland code for each of the twins occupational fantasy and leisure pursuits.

Scores on the Iachan (1990) Index range from 0-6 (the higher the number, the greater the congruence). A score of six indicates the two-letter codes are identical (e.g., RI and RI); a score of five signifies the two codes have identical first letters but different second letters (e.g., AS and AI). At the other extreme, scores of O indicate the two codes are totally different (e.g., SE and AR) and a score of 1 signifies they have different primary codes but the same secondary code (e.g. EA and CA). A score of 4 indicates that the two codes have identical letter but the order is reversed (e.g., AI and IA). A score of 2 signifies only one letter is common to the two codes and the ordinal position of the letter is different (e.g., SC and ES). A score of 3 is not possible. All two-letter codes were determined by using The Leisure Activities Finder (Holmberg, Rosen, & Holland, 1990). An index score of 2 was calculated for their occupational fantasy (e.g. veterinarian and animal training) and an index score of 6 for their leisure pursuit (e.g., both mentioned "watching movies").


Holland (1997) noted that the major heritability studies using twins strongly suggest that vocational interests have a much larger inherited component than earlier studies suggested (Reardon & Lenz, 1998). This research on twin studies and vocational interests adds another level of explanation for the stability of vocational interests and, by implication, the stability of work histories (Holland, 1997). Of course, the question of whether the high congruence found in our identical twins is a result of primarily genetic or environmental factors remains unanswered. Still, the results appear to reinforce commonsense notions about the role of genetics and environments in one's development.

This study is not without limitations. The data used in this study is based on only one set of identical twins. In addition, the results are based on only one instrument: the Self-Directed Search. Even given these shortcomings, the results seem to suggest that there exist multiple determinants of vocational interests. To extend the findings of this study, researchers should include fraternal twins and--if possible-identical twins reared apart from early childhood.


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Assistant Professor, Counseling

Louisiana Tech University


Assistant Professor, Psychology and Behavioral Sciences

Louisiana Tech University


Professor, Counseling

Louisiana Tech University

Psychology and Behavioral Sciences


Vice President for Student Affairs

Louisiana Tech University


Graduate Candidate, Counseling

Louisiana Tech University
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Author:Chauvin, Ida; McDaniel, Janelle R.; Miller, Mark J.; King, James M.; Eddlemon, Ondie L.M.
Publication:College Student Journal
Article Type:Report
Geographic Code:4EUNE
Date:Mar 1, 2012
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