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National Institute on Drug Abuse
Women and Sex/Gender Differences Research
Director's Report to Council
Research Findings Excerpts
May, 1998
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Basic Research |
Cocaine & Female Reproductive Function
NIDA supported research findings published in the January 1998 issue of The American Journal of Obstetrics and Gynecology which reported on the cocaine-induced disruption in menstrual and ovarian cyclicity in monkeys. These physiological changes were observed in monkeys following daily administration of cocaine during the normal cycling (follicular-phase) period. The disruption in menstrual cyclicity and folliculogenesis were independent of weight loss, caloric intake, or basal gonadotropin levels. Potter, D.A., et al, Amer. J. Obstet. Gyn. 178, pp. 118-125, 1998.
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AIDS Research |
Assessing the Impact of HIV Risk Reduction Counseling
Changes in cognitive, psychological, and risky behavior latent variables after Traditional or Specialized AIDS education was assessed after two years using structural equation modeling (SEM) in a sample of impoverished at-risk African-American women (N=300). Both groups reported significant improvement at 2 years in their Self-Esteem and Social Resources. They also reported less Threat Perception, Avoidant Coping, Emotional Disturbance, HIV Risk Behavior, and Drug Use Behavior. There was an advantage to Specialized group membership. When compared to the Traditional group at two years, women in the Specialized group reported enhanced social resources, reduced emotional distress, less use of an avoidant coping style and less drug use. Advantages of culturally sensitive HIV risk reduction program and the importance of connecting women with social services available in their communities is discussed. Nyamathi, A.M., and Stein, J.A. AIDS Education and Prevention, 9(3), pp. 253-273, 1997.
Study Identifies Sexual Risk Behaviors among Native American Drug Users
This study describes patterns of sexual behavior and condom use in a sample of Native American drug-using men and women (N=114). Data were collected on self-reported sexual behaviors in the past 30 days, including descriptions of the most recent sex partners (up to five partners). There were 157 sex partner pairs, of which at least one partner was a drug user. Native American women (55%) were more likely than Native American men (23%) to report never using condoms for vaginal and anal sex in the last 30 days. Compared with other ethnic pair combinations, sex partner pairs composed of Native American women and white men (n=18) were least likely to use condoms (6% of pairs) and the most likely to report an injection drug user sex partner (33% of pairs). These results suggest a potential vector of HIV and other STDS between white male IDUs and Native American women and highlight the need for further qualitative and quantitative research to examine the factors underlying this pattern of sexual risk behavior. Fenaughty, A.M., et al. Sex Partners of Native American Drug Users. Journal of AIDS & Human Retrovirology, 17(3); pp. 275-282, 1998. HIV-Risk Behaviors and Mental Health Characteristics Among Homeless or Drug Recovering Women and their Supportive Person This paper describes risky drug and sexual behavior and mental health characteristics in a sample of 240 homeless or drug-recovering women and their most immediate source of social support. Women and their closest support sources both reported a great deal of recent non-injection drug use (56% and 52%, respectively) and lesser, though similar, amounts of recent injection drug use (12% and 14%, respectively). Over a third of both groups reported a history of a STD and sexual activity with multiple partners. Fifty one percent of the women and 31% of their companions had CES-D scores of 27 or greater, suggesting a high level of depressive disorders in both samples. Similarly, 76% of the women and 59% of their friends had psychological well-being scores below a standard clinical cutpoint. These data suggest that homeless and impoverished women are turning to individuals who are themselves at high risk for emotional distress and risky behaviors as their main source of support. Implications relating to the importance of integrating the dyad in interventions and introducing alternate sources of support are discussed. Nyamathi, A., Flaskerud, J., and Leake, B. Nursing Research, 46(3), pp. 133-137, 1997.
Relative Impact of Two AIDS Education Programs among High-Risk
Women on Cognitive, Behavioral, and Psychosocial Variables Changes in cognitive, psychological, and risky behavior latent variables were assessed after traditional or specialized AIDS education after 2 years using structural equation modeling in a sample of impoverished at-risk African American women (N = 300). The traditional group watched an AIDS videotape and received a 1-hour basic AIDS education program. In addition to the videotape, the specialized group received a 2-hour program in which they received a demonstration of risk-reducing behaviors, discussion of problem-focused coping, and techniques to enhance self-esteem. Also, they received individualized responses to their concerns such as referrals to drug rehabilitation programs or shelters. Both groups reported significant improvement at 2 years in their self-esteem and social resources. They also reported less threat perception, avoidant coping, emotional disturbance, HIV risk behavior, and drug use behavior. When compared with the traditional group at 2 years, women in the specialized group reported more social resources, more reduced emotional distress, less use of an avoidant coping style, and less drug use. The advantages of culturally sensitive HIV risk reduction programs and the importance of connecting women with social services available in their communities is discussed. Nyamathi, A.M., and Stein, J.A. AIDS Education and Prevention, 9, pp. 253-273, 1997.
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Epidemiology, Etiology, and Prevention |
Association Between a Microsatellite Polymorphism at the DRD5 Gene and the Liability to Substance Abuse
This population-based study of the relationship between the risk for substance abuse and the dopamine D5 receptor locus (DRD5) was conducted in a sample of adult males and females with a DSM-III-R diagnosis of substance dependence (SD) or without any psychiatric disorder who are participants in a longitudinal family/high-risk study of substance abuse (Center for Education and Drug Abuse Research, CEDAR). To lessen the potential influence of the stratification bias, only European-American subjects' DNA samples were used in this pilot study. To maximize the phenotypic differences for the liability to substance abuse between the affected and nonaffected subsamples, the control subjects were selected to be older than 35 years of age (males' age range, 35-64; females', 35-52), and thus had predominantly been beyond the age of risk for development of SD. The data showed an increased frequency of the modal, 148 base pairs repeat length, allele of a dinucleotide polymorphism (allele 9) among SD males. This finding was reproduced in females with even greater contrast between affected and nonaffected individuals. The association was also found to be significant when all the alleles of the polymorphism were taken into account in a likelihood ratio test. The case-control and sex differences were also observed for genotypes coded in accordance with the number of alleles 9 (0 to 2). It is noteworthy that none of the SA females had the non-9/non-9 genotype, while the frequency of this genotype among control females was 37%; among males, the respective frequencies were 25 and 47%. The genotype was also found to be associated with novelty seeking in females, but not in males, in whom even a trend for such an association was absent. Interestingly, there were no sex differences in novelty seeking among substance abusers or controls, while the relationship between this temperament/personality trait and the risk for SD was more pronounced in males than females. The findings suggest that the DRD5 locus is involved in the variation and sex dimorphism of the liability to substance abuse and related traits. Vanyukov, M.M., Moss, H.B., Gioio, A.E., Hughes, H.B., Kaplan, B.B., and Tarter, R.E. Behavior Genetics, 28, pp. 75-82, 1998.
Using the National Youth Survey Data Socioeconomic Status Has a Nonlinear Relationship with Marijuana Use
This may account for many null findings that used linear methods to describe the relationship. This study also found that predictors for females were substantially different from males. However, weekly alcohol use was the strongest predictor for both groups (both odds ratios greater than 11.0). Other predictors for both sexes included having a job, having friends who use marijuana and having used some alcohol in the past year. For females, prior victimization and low school aspirations were also significant. For males, GPA, commitment to friends, urbanicity, time spent with friends and peer strain were also significant predictors. Miller, D.S., and Miller, T.Q. Addictive Behaviors, 22(9), pp. 479-489, 1997.
Tobacco Use Among Mexican American Youth
Increases in smoking/tobacco-related diseases among the Hispanic population call for an examination of tobacco use among this population. This study examined the relationship between gender, level of cultural identification, migrant status, grade level, tobacco use, and perception of harm among Mexican American youth. Results showed males were more likely to use cigarettes (occasional and daily) and smokeless tobacco than females when grade, cultural identification, and migrant status of parent are held constant. No gender effect was found for lifetime cigarette use. The odds of using cigarettes and smokeless tobacco were found to increase substantially across grades. Effects were found for Mexican American/Spanish and Anglo/White American cultural identification and daily cigarette use. Youths who belonged to nonmigrant families or who identified with a traditional Mexican American/Spanish culture were more likely to consider the regular use of tobacco harmful. Casas, M.J., Bimbela, A., et. al. Cigarette and Smokeless Tobacco Use Among Migrant and Nonmigrant Mexican American Youth. Hispanic Journal of Behavioral Sciences, 20(1), pp. 102-121, 1998.
Ethnicity and Gender in Adolescent Drug Resistance
While ethnic and gender differences have been reported in the amount and type of drug use, little is known about how drugs get offered to adolescents and how members of different cultural groups respond to those offers. Interviews (30-40 minutes) were conducted with 158 middle school students. Adolescent "near-peer" interviewers were trained for this project and matched to participants by ethnicity and gender. Data were content analyzed and results showed that for all groups, simple offers were more likely than complex, pressure-filled strategies, and these offers were typically resisted through simple refusals (i.e., "no") rather than more complex techniques (e.g., explanations). Drugs tended to be most frequently offered to all groups at home or in public rather than at school or parties. Gender and ethnic differences were observed in other areas. Latinos/Latinas were significantly more likely to experience drug offers than other males and females. Hecht, M., Trost, M., Bator, R., & MacKinnon, D. Ethnicity and Gender Similarities and Differences in Drug Resistance. Journal of Applied Communication Research, 25, pp. 1-23, 1997.
Survey Finds Parenting Differences Between Narcotic Addicts and Their Parents
A 2-part survey was administered to male and female narcotic addicts receiving methadone maintenance to examine their retrospective self-reports about how they were parented and compare them with their views about their own parenting practices of their adolescent children. The sample of 313 adult addicts included 248 females (79%) and 65 males (21%). The average age was 37.6 years. There were 250 African Americans (80%) and 63 Whites (20%) in the sample. The mean age of addiction (narcotic use 4 days a week for a month or more) was 22.5 years, and the average period of continuous use was 5.4 years. Since the age of addiction, the subjects had spent an average of 10.7 months in prison. The addicts viewed their mothers as significantly more effective at parenting than their fathers, especially in terms of interaction, communication, and affection. Fathers were generally perceived as unavailable and distant. In terms of their own parent practices, the addicts reported being significantly more involved, more responsible, and more closely attached to their children than their parents were to them. They also reported that they were more active disciplinarians and utilized significantly fewer punitive behaviors with their children. Nurco, D.N.,et al., The Family Experiences of Narcotic Addicts and Their Subsequent Parenting Practices. Am. J. Drug Alcohol Abuse, 24(1), pp. 37-59, 1998.
Adolescent School Experiences and Dropout, Adolescent Pregnancy, and Young Adult Deviant Behavior
Predictive effects of school experiences were studies over a 7-year interval in a random community sample of 452 adolescents, 12 through 18 years of age. Outcomes examined included dropping out of school, adolescent pregnancy, engaging in criminal activities, criminal conviction, antisocial personality disorder, and alcohol abuse. Logistic regression showed academic achievement, academic aspirations, and learning-focused school settings to be related to a decline in deviant outcomes independent of the effects of disadvantaged socioeconomic background, low intelligence, childhood conduct problems, and having deviant friends during adolescence. Associations between school conflict and later deviancy were mediated by deviant peer relationships in adolescence and other school characteristics. Prior research reporting continuity of childhood conduct problems and the influence of adolescent affiliations with deviant peers on negative outcomes was supported. Implications for using the school context in risk factor research and the practical applications of such research for intervention are discussed. Kasen, S., Cohen, P., and Brook, J.S. Journal of Adolescent Research, 13(1), pp. 49-72, 1998.
Factors Associated with Early Sexual Activity among Urban Adolescents
This study uses lifespan and ecological frameworks to investigate the factors associated with early adolescent sexual activity. Data from a longitudinal study of urban teenagers of color address three issues: (1) the prevalence and pattern of sexual activity among boys and girls ages 15 and younger, (2) the link between early sexual activity and high-risk sexual behavior, and (3) the life contexts linked with early sexual activity. Results from 803 African American and Hispanic adolescents suggest a high prevalence of early sexual activity, which is associated with higher rates of childbearing and risky sexual behavior than sexual activity initiated in later adolescence. Somewhat different factors are associated with early sexual activity for boys and girls, although family composition, parent attachment, and substance use are important for both genders. Implications for intervention are discussed. Smith, C.A. Social Work, 42, pp. 334-346, 1997.
Patterns by Gender and Ethnicity Among School Attenders and Dropouts
Differences in patterns of volatile solvent use were explored with special emphasis on use as related to school enrollment status. The sample included American Indian, Mexican-American and White American youth. Three enrollment status categories were identified: dropout, academically at-risk (enrolled), and control. A self report survey was used to assess both level and intensity of volatile solvent use. Findings indicated that a higher proportion of the dropout cohort have used volatile solvents, used volatile solvents regularly, and used volatile solvents with more intensity than either the academically at-risk group or the control group. An interaction between gender and ethnicity was also revealed; American Indian females reported higher lifetime prevalence and thirty-day prevalence than did American Indian males, whereas for both the Mexican-American and White American samples, males report higher rates than females. Findings are discussed in terms of the influence of volatile solvent abuse and school success as well as previous findings. Bates, S.C., Plemons, B.W., Jumper-Thurman, P. Beauvais, F. Patterns by Gender and Ethnicity Among School Attenders and Dropouts. Drugs and Society, 10, pp. 67-78, 1997.
Effect of Parental Mental Health Status on Adolescents' Dietary Behaviors
In a study by NORC, investigators examined whether adolescents of substance-abusing and depressed parents were more likely to have poor dietary behaviors that those in the health comparison families the examined 841 adolescents in families of substance-abusing parents, depressed parents, and parents without diagnosable psychiatric disorder. All adolescents were given a food frequency questionnaire. Adolescents whose parents had substance abuse disorder had lower intakes of fruits and higher intakes of high fat foods, and also ate more frequently at fast-food restaurants and purchased more snacks. Adolescents whose parents were depressed had lower intakes of all food groups. Mothers' mental health status impacted more on adolescents dietary behaviors than did the father's mental health status. This research suggests that at-risk behaviors among youth of psychiatrically impaired parents may extend to food behaviors. Su, L.J., Story, M., and Su., S.S. Journal of Adolescent Health, 20, pp. 426-433, 1997.
Drug-User Treatment Programs in a Large Metropolitan Area
In 1993/1994 a compre- hensive survey of the Los Angeles County drug-user treatment systems was undertaken. The system contains more than 300 programs covering over 4,000 sq. miles and serves clients from a diverse population of over nine million people. Overall, 58% of the 14,860 clients being served at the time of the survey were male; about 35% were White, 34% Latino/Latina, 26% African-American, 2% Asian/Pacific Islander, 0.5% Native American and 2.5% of unknown ethnicity. Almost all programs reported a smaller number of current clients than allowed by maximum capacity, indicating that waiting lists were not necessarily related to lack of treatment slots, but more a lack of affordable (i.e., publicly-funded or insurance-covered) treatment slots. Waiting times ranged from one day to over one year. Polinsky, M.L., Hser, Y.I., Anglin, M.D., and Maglione, M.A. Subs. Use & Misuse, 33(8), pp. 1739-1765, 1998.
Consideration of Special Populations in the Drug Treatment System of a Large Metropolitan Area
Findings are based on a recent comprehensive survey of 294 drug treatment programs in Los Angeles County. Special populations were grouped by health status, ethnic background, language needs, and gender-related needs (groups are not mutually exclusive.) Survey results indicated a generally high proportion of programs capable of meeting the unique needs of a variety of special population clients and most programs have some mix of special population clients in their current caseload. About 62% of programs reported being able to serve clients who were primarily Spanish speakers. Clients using American Sign Language could be served by 11% of programs. About 40% of programs reported not being able to serve pregnant women, including almost half of the hospital inpatient programs and a quarter of the outpatient drug-free programs. Although almost 70% of programs reported being able to serve clients with mobility impairments, only 26% of the residential programs reported this capacity. Polinsky, M.L., Hser, Y.I., and Grella, C.E. Journal of Behavioral Health Services & Research, 25(1), pp. 7-21, 1998.
Substance Use among Nurses: Differences between Specialties
Valid data on factors that increase a health care worker's likelihood of substance use are integral in assuring professional standards and quality health care for consumers. This study explored the association between nursing speciality and past-year substance use. In an anonymous mailed survey, a balanced stratified sample of registered nurses (n=4438) reported their use of marijuana, cocaine, and prescription-type drugs, as well as cigarette smoking and binge drinking. Prevalence of use of all substances was 32%. Rates varied by specialty, even when sociodemographics were controlled. Compared with nurses in women's health, pediatrics, and general practice, emergency nurses were 3.5 times as likely to use marijuana or cocaine (odds ratio [OR]=3.5, 95% confidence interval [CI]=1.5, 8.2), oncology and administration nurses were twice as likely to engage in binge drinking; and psychiatric nurses were most likely to smoke (OR=2.4, 95% CI=1.6, 3.8). No specialty differences appeared for prescription-type drug use. These data suggest that certain nursing specialities were more likely than others to be associated with substance use. The differences were not explained by demographic characteristics. Inasmuch as a comparison of these results for nurses with prior work on physicians found considerable agreement by speciality, preventive initiatives should consider interdisciplinary approaches to substance use education. Trinkoff, A.M., and Storr, C.L. Amer J Pub Health, 88, pp. 1-5, 1998.
General Deviance and Psychological Distress: Impact of Family Support/Bonding over 12 Years from Adolescence to Adulthood
Comorbidity occurs within and across various domains of human pathology and may be diverse manifestations of a single, general dysfunction in early family support and bonding. Family socialization, pseudo maturity, and self-derogation theories were tested using cross-sectional and 12-year prospective data from a community sample assessed in late adolescence (age 18) and again in adulthood (age 30). All of the hypotheses and expected findings received some support in the data analyses these confirmed that: general deviance and psychological distress were significantly correlated for both men and women and therefore are overlapping and comorbid disorders; both general distress and psychological distress were significantly predicted by family support/bonding fully accounted for the cross-sectional association between general deviance and psychological distress for the women and general deviance for the men: both theories of pseudo maturity and self-derogation explained many of the prospective effects from late adolescence into adulthood: sexual involvement, although an indicator of general deviance, related negatively with indicators of psychological distress; different patterns were evident for the development periods of adolescence compared with adulthood; and many of the processes differed by sex. Newcomb, M.D. Criminal Behavior and Mental Health, 7, pp. 369-400, 1997.
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Clinical and Services Research |
Attention Deficit Disorder among Substance Abusers
Howard Schubiner, M.D. and his colleagues at Wayne State University School of Medicine have been studying the prevalence of Attention Deficit Hyperactivity Disorder (ADHD) in a cross-section sample of adults seeking substance abuse treatment. ADHD is considered to be a genetic and neurobiological disorder with a 5% prevalence rate in all children within the U.S. In a sample of 201 adult substance abusers (106 males and 95 females) 24% were found to meet DSM-IV clinical criteria for ADHD (as a child and as an adult). The gender prevalence in males was 28% and 19% in females, a non-significant difference. Overall, substance abusers with ADHD (compared to substance abusers without ADHD) were more likely to have conduct disorder and antisocial personality disorder (ASPD) and more motor vehicle accidents. Gender effects were noted. While the use of specific types of drugs was not significantly different between the groups of substance abusers with and without ADHD, female substance abusers with ADHD had an increased number of treatments for alcohol abuse and dependence. (The above findings were presented at the at the Fifth Annual Conference on Behavior, Neurobiology, Substance Abuse, and Culture, on October 16, 1997, in Los Angeles, California).
Menstrual Cycle Effects on Tobacco Cessation Symptoms
Dr. Sharon Allen at the University of Minnesota, has conducted a study to examine the effects of the menstrual cycle on smoking behavior in women during continued smoking and quit smoking status. Withdrawal and premenstrual symptomatology during phases of the menstrual cycle were investigated. Preliminary analysis indicated that there were no significant differences in withdrawal symptom scores across menstrual cycle phases in both the continued smoking and quit smoking groups. As expected, premenstrual symptom scores tended to be higher in the late luteal phase compared to the follicular phase in both the continued smoking (p=.03) and quit smoking (p=.0014) groups. Although total withdrawal symptom scores do not differ significantly across cycle phases, these findings suggest that smoking cessation may be more difficult in the late luteal phase due to increased premenstrual symptoms. Christianson, D., Allen, S., Hatsukami, D., Nelson, D. Smoking Withdrawal and Premenstrual Symptomatology Across Menstrual Cycle Phases. Presented at the Society for Research on Nicotine and Tobacco. March, 1998.
Efficacy of Lithium Treatment for Adolescents With Bipolar Disorders and Secondary Substance Dependency
Dr. Barbara Geller and colleagues at the Washington University School of Medicine conducted a 6-week double-blind, placebo-controlled, parallel group clinical trial to assess the efficacy of lithium for 16 male and 9 female adolescents (12-18 years) with a primary DSM-III-R bipolar disorder and secondary substance dependency disorder. Results indicate lithium to be an efficacious treatment for both disorders in terms of concurrently reducing symptoms of psychopathology (e.g., improved mood) and the use of illicit substances. No gender differences were noted. Geller, B., Cooper, T.B., Sun, K., et al. Journal of the American Academy of Child and Adolescent Psychiatry, 37(2), pp. 171-178, 1998.
Validity of Substance Use Self-Reports in Dually Diagnosed Outpatients
Dr. Roger Weiss and colleagues at the Department of Psychiatry, Harvard Medical School compared the self reports of substance use with supervised urine samples collected on the same day for subjects participating in two separate studies, each designed to test a new group psychotherapy for patients with coexisting substance use disorder and with either PTSD or bipolar disorder. The mean age of the subjects was 35.2 years. All subjects in the PTSD group were women, by design, and 53.8% of the subjects in the bipolar group were men. Most patients, 69% had both alcohol and drug dependence and 95.6% were dependent on more than one drug. Marijuana and cocaine were the most common primary drugs of abuse. Results indicate that self reports were highly valid. Only 4.7% of cases involved subjects not reporting substance use detected by urine screens. Weiss, R., Najavits, L., Greenfield, S., Soto, J., Shaw, S., Wyner, D. American Journal of Psychiatry, 155, pp. 127-129, 1998.
A Discrete Choice Model of Drug Abuse Treatment Location
Goodman and his associates used three years of insurance claims for 9,878 individuals representing 10 large self-insured firms to examine determinants of inpatient care for disorders related to drug abuse. In addition to examining the effects of individual characteristics on treatment utilization, the research focused on the ways in which differences among firms affect the choice of treatment location. The probability of inpatient drug treatment was found to be related to diagnosis of a cocaine problem (abuse or dependence), a diagnosis of drug dependence (versus abuse), and a diagnosis of psychosis. Men were more likely to be hospitalized than women. Controlling for patient risk factors, where patients are employed and have insurance made a substantive difference in the probability of receiving inpatient treatment, with the firm-specific probability varying by as much as 87 percent. Over time, a trend toward more use of outpatient treatment was observed. Goodman, A.C., Nishiura, E. & Hankin, J.R. A Discrete Choice Model of Drug Abuse Treatment Location. Health Services Research, In Press.
Drug Treatment Outcomes and Physical and Sexual Abuse
Relationships between psychopathology, drug treatment outcomes, and history of physical or sexual abuse were examined in a longitudinal study of 330 patients in 26 outpatient programs. Both forms of abuse were found to be associated with higher levels of psychopathology, but with significant gender differences. For women in the study, sexual abuse was associated with higher levels of psychological disturbance, while physical abuse was associated with less disturbance. The inverse relationship was found for men: physical abuse was associated with several types of psychological disturbances, while sexual abuse was associated only with anxiety disorder. The study points to the need to assess histories of sexual and physical abuse in assessing and treating co-occurring psychopathology. Gil-Rivas, V., Fiorentine, R., Anglin, M.D., and Taylor, E. Sexual and Physical Abuse: Do they Compromise Drug Treatment Outcomes? Journal of Substance Abuse Treatment, 14, pp. 351-358, 1997.
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Intramural Research |
Cocaine Binds Directly to Human Hair: In Vitro Characterization
Previous studies at the IRP have shown that human hair can sequester cocaine. However, detailed binding studies examining the complex nature of binding of cocaine to human hair have never been carried out. The effect of differences between hair types on the extent of drug incorporation into hair is an important issue, because hair is currently being collected and analyzed to identify drug use by individuals in the workplace. A major concern is that hair test results may not be consistent or impartial for individuals with different hair types. Therefore, an understanding of the nature of drug binding to hair is needed in interpreting hair testing results. In this study investigators carried out the in vitro characterization of binding of radiolabeled cocaine to human hair. Results indicate that human hair can bind cocaine directly in a saturable manner with a stereoselectivity favoring the levoisomer of cocaine. Greater radioligand binding occurred in male Africoid hair than in female Africoid hair and in all Caucasoid hair types. Melanin was considered the most likely binding site for cocaine in hair. Differences in radioligand binding between hair types appeared to be due to differences in the density of binding sites formed by melanin in hair. Joseph, R.E., Tsai, W.J., Tsao, L-I, Su T-P and Cone E.J. J. Pharmacol. Exp. Ther., 282, pp. 1228-1241, 1997.
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