Age of initiating selected health-risk behaviors among high school students in the united states

JOURNAL OF ADOLESCENT HEALTH 1997.:21:225-231

ORIGINAL ARTICLE

Age of Initiating Selected Health-Risk Behaviors Among High School Students in the United States CHARLES W. WARREN, Ph.D., L A U R A K A N N , Ph.D., MEG LEAVY SMALL, Ph.D., J O H N S. SANTELLI, M.D., JANET L. COLLINS, Ph.D., A N D LLOYD J. KOLBE, Ph.D.

Purpose: To estimate and compare the age of initiation of alcohol use, cigarette smoking, sexual intercourse, and marijuana use among female and male students in U.S. high schools. Methods: Using data from the 1991 and 1993 national school-based Youth Risk Behavior Surveys, life-table analysis was used to create hypothetical cohorts to estimate age of initiation of selected health-risk behaviors. The sample size was 12,272 in 1991 and 16,296 in 1993, with an overall response rate of 68% in 1991 and 70% in 1993. Results: Male students initiate each of these behaviors earlier than female students, but the pace of initiation for females accelerates so that by age 15 years the cumulative proportion of male and female students who have initiated these behaviors is similar. For both female and male students, the youngest cohort appears to initiate use of alcohol and sexual intercourse at earlier ages than older cohorts. Similarly, the younger cohorts of female students appear to initiate smoking cigarettes and using marijuana at earlier ages than older cohorts. Conclusions: Many high school students are initiating alcohol use, cigarette smoking, sexual intercourse, and marijuana use at early ages. These data suggest a need for intensive intervention programs by middle/junior high school to motivate and prepare students to avoid these behaviors. Clinicians should begin screening and counseling for risk behaviors in early adolescence. © Society

for Adolescent Medicine, 1997

From the Division of Adolescent and School Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia Address reprint requests to: Dr. Charles W. Warren, Division of Adolescent and School Health, Centers for Disease Control and Prevention, Mailstop K-33, 4770 Buford Highway, Atlanta, GA 30341-3724. Manuscript accepted March 28, 1997.

KEY WORDS:

Adolesence Risk behaviors Age of initiation Smoking / alcohol Sexual intercourse Marijuana Gender differences

Adolescents who initiate health-risk behaviors such as sexual intercourse and substance use at an early age frequently have poorer health later on in life, lower educational attainment, and less economic productivity than their peers (1,2). Early initiation of these behaviors is associated with longer periods of risk taking in later adolescence and early adulthood and also may be a marker for risk tatting in adulthood (1,3-5). Timely targeting of health promotion including screening and counseling programs for young people requires information on the ages at which risky behaviors are initiated as preventing the initiation of a behavior may be easier than eliminating a behavior once adopted. For example, a number of programs have successfully prevented tobacco use among adolescents (6), but smoking cessation initiatives for this age group have not been as successful (7). Similarly, programs to encourage sexual abstinence and thereby prevent unintended pregnancy and sexually transmitted diseases tend to be most effective among adolescents who have not yet initiated sexual intercourse (8). Many researchers have attempted to determine the ages at which young people initiate alcohol use (2,9-11), cigarette smoking (10,12-13), sexual inter-

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course (5,14-16), and marijuana and other drug use (10). However, most of these studies have been limited to a single risk behavior, focusing on a select population (e.g., a restricted age group, a single sex, or a non-national sample) or using a static time frame (i.e., not following the study group over time, either retrospectively or prospectively). In this study, we used the 1991 and 1993 national school-based Youth Risk Behavior Surveys (YRBS) to examine the age of initiation of four risk behaviors: alcohol use, smoking a whole cigarette, sexual intercourse, and use of marijuana, among nationally representative samples of female and male students in grades 9-12 in the United States. We sought answers to the following questions: (a) At what age do adolescents initiate these risk behaviors? (b) Does age of initiation differ between female and male students? and (c) Is the age of initiation becoming earlier over time, i.e., are current high school students engaging in these behaviors earlier than did their predecessors?

Methods In 1989, the Centers for Disease Control (CDC) developed the YRBS questionnaire in collaboration with representatives of 19 other federal agencies, numerous state and local departments of education, and academic experts in each of six categorical health risk behaviors (17). The questionnaire measures important health-risk behaviors such as tobacco use, alcohol and other drug use, sexual activity, behaviors that increase the risk of injury, dietary practices, and physical activity. There were 75 questions in 1991 and 87 in 1993. The questionnaire, which was designed to be self-administered in a classroom setting, has a seventh-grade reading level and contains multiple choice questions which elicit self-reported data appropriate for estimating the prevalence of health risk behaviors among adolescent populations. Extensive laboratory and field tests were conducted to improve the wording of questions, determine the most appropriate recall periods, and identify response options. A test-retest reliability study of the YRBS found that kappa values for the age of initiation questions used in this study ranged from 68% to 71% (18). Survey procedures allowed students to respond anonymously; yet, underreporting or overreporting of behaviors may have occurred, depending, in part, on the perceived social stigma or support for the behaviors and the perceived confidentiality of the responses (17). Studies to validate the questions on dietary practices and physical activ-

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ity are under way; as has been done for the questions on alcohol use (19). In 1991 and 1993, the YRBS used independent, cross-sectional, three-stage cluster samplLes of 9ththrough 12th-grade students in public and private schools in all 50 states and the District of Columbia. Schools with high percentages of black and Hispanic students were sampled at a higher rate than other schools. Within sampled schools, one or two intact 9th- through 12th-grade classes of a required subject, such as English or social studies, were selected at random to participate in the survey. Sample sizes were 12,272 in 1991 and 16,296 in 1993; the school response rates were 75% and 78%, respectively; the student response rate was 90% in both years, and the overall response rates were 68% and 70%, respectively. Data were edited for inconsistency, and a weighting factor was applied to each student record to adjust for nonresponse and oversampling of black and Hispanic students. The weights were scaled to make the weighted count of students per grade equal the number of students in that grade. The age of initiation questions in the 1991 and 1993 YRBS were each worded similarly (i.e., "How old were you when you...?"). The response categories were of two general types. The questions on first time drinking alcohol other than a few sips, smoking a whole cigarette, and trying marijuana used the following response categories: (a) "I have never tried..."; (b) "<9 years old"; (c) "9 or 10 years old"; (d) "11 or 12 years old"; (e) "13 or 14 years old"; (f) "15 or 16 years old"; and (g) "->17 years old." In contrast, the question on initiation of sexual intercourse used these categories: (a) "I have never had sexual intercourse"; (b) "<12 years old"; (c) "12 years old"; (d) "13 years old"; (e) "14 years old"; (f) "15 years old"; (g) "16 years old"; and (h) "->17 years old." We applied life-table procedures to the responses to calculate the cumulative probability of initiating the four risk behaviors at specific ages by sex, and by sex and current age. Life tables are mathematical models that start with a hypothetical cohort of persons at a specified exact age x, then follow the progression of that cohort as an event (e.g., a behavior) is initiated between exact age x and exact age x + n. For example, the probability of initiating smoking between ages 11 and 13 years would be the proportion of persons in the cohort at age 11 years who had initiated smoking before reaching age 14: years. Per life-table procedures, we calculated the median age of initiation of a behavior as the age at which 50% of

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Table 1. Cumulative Proportion of High School Students Who H a d Initiated Alcohol Use, Smoked a Whole Cigarette, Used Marijuana, and H a d Sexual Intercourse at Each Exact Age and Median Age of Initiation, by Gender, 1991 and 1993 Youth Risk Behavior Survey 1991 Age (years) First initiated alcohol use 9 11 13 15 17 Median age First smoked whole cigarette 9 11 13 15 17 Median age First initiated sexual intercourse 12 13 14 15 16 17 Median age First initiated marijuana use 9 11 13 15 17 Median age

1993

Total

Female

Male

Total

Female

Male

10.6 (-+1.1)a 18.2 (+2.6) 32.7 (_+2.8) 62.8 (_+2.4) 84.0 (_+2.9) 14.1 (_+0.1)

7.7 (+-0.8) 13.6 (_+1.2) 27.8 (_+1.8) 59.5 (_+2.2) 83.5 (_+2.8) 14.4 (_+0.1)

13.4 (_+3.2) 22.6 (_+3.8) 37.4 (_+4.6) 65.9 (_+4.6) 84.4 (_+3.8) 13.9 (_+0.1)

11.2 (_+1.2) 18.7 (_+1.3) 32.8 (_+1.9) 61.9 (_+2.2) 83.7 (_+1.6) 14.2 (_+0.1)

8.5 (_+0.4) 14.3 (+0.8) 27.0 (_+1.0) 59.6 (_+1.4) 83.8 (+1.6) 14.4 (_+0.1)

13.8 (_+1.6) 22.8 (--2.0) 38.1 (_+2.2) 64.1 (_+2.2) 83.5 (_+1.8) 13.9 (_+0.1)

4.8 (_+1.3) 11.5 (_+2.0) 23.8 (_+2.7) 43.1 (_+3.1) 58.0 (_+3.2) 15.9 (+0.1)

3.7 (-+1.5) 10.0 (+2.1) 22.1 (_+3.0) 42.7 (_+3.6) 58.2 (_+3.9) 15.9 (+-0.1)

5.7 (-+1.2) 12.9 (-+2.0) 25.4 (_+2.5) 43.4 (_+2.8) 57.7 (_+2.8) 15.9 (_+0.1)

4.9 (_+0.8) 12.8 (_+1.3) 26.9 (_+1.8) 45.5 (+2.1) 60.2 (_+2.1) 15.6 (_+0.1)

3.5 (_+0.8) 9.4 (_+1.3) 23.2 (_+1.9) 43.9 (_+2.3) 59.6 (_+2.3) 15.8 (+-0.1)

6.3 (-+0.9) 15.8 (_+1.4) 30.1 (_+1.8) 47.0 (-+2.0) 60.8 (+_2.0) 15.4 (+-0.1)

5.6 (_+1.4) 10.2 (-+1.9) 17.2 (_+2.2) 28.3 (_+2.6) 42.7 (_+3.1) 58.6 (_+3.4) 16.5 (_+0.1)

1.9 (-+0.9) 5.1 (_+1.4) 10.9 (_+1.8) 22.6 (_+2.5) 39.0 (_+3.0) 56.2 (_+3.4) 16.6 (_+0.1)

9.2 (_+2.3) 15.0 (_+2.6) 23.3 (_+3.1) 33.7 (_+3.4) 46.3 (_+3.6) 60.9 (_+3.6) 16.3 (_+0.1)

4.7 (_+1.5) 9.2 (_+1.7) 16.5 (_+1.6) 28.3 (_+1.9) 43.2 (-+2.3) 58.7 (_+2.7) 16.4 (_+0.1)

1.7 (+-0.6) 4.2 (_+1.0) 10.4 (_+1.3) 23.1 (_+1.7) 39.7 (_+2.1) 56.9 (_+2.5) 16.6 (_+0.1)

7.5 (_+2.0) 13.8 (_+2.6) 22.2 (_+2.9) 33.1 (_+3.2) 46.4 (-+3.5) 60.4 (-+3.5) 16.3 (_+0.1)

1.3 (_+1.2) 3.0 (_+1.1) 7.4 (_+1.6) 19.2 (_+2.5) 34.3 (-+3.5) NA

0.8 (_+0.5) 1.8 (_+1.0) 5.3 (_+1.2) 17.7 (_+2.2) 33.1 (_+3.2) NA

1.7 (+1.1) 4.2 (_+1.3) 9.4 (+_2.2) 20.6 (_+3.0) 35.5 (_+3.8) NA

1.4 (+-0.7) 2.8 (_+1.1) 6.9 (+_1.8) 18.3 (_+2.4) 36.0 (_+3.4) NA

0.8 (+-0.6) 1.6 (+-0.9) 4.3 (_+1.5) 14.6 (_+2.4) 32.7 (_+3.3) NA

1.9 (_+0.8) 3.8 (-+1.4) 9.3 (_+2.0) 21.8 (_+2.5) 39.1 (-+3.4) NA

95% confidence interval. NA = not applicable; <50% of the population had used marijuana. the cohort had experienced the event. We included in the analysis respondents who had never initiated the b e h a v i o r u p to t h e a g e at w h i c h t h e y w e r e s u r v e y e d .

Results Table I shows (by gender) the cumulative proportion of h i g h s c h o o l s t u d e n t s w h o r e p o r t e d e n g a g i n g in s p e c i f i c h e a l t h - r i s k b e h a v i o r s at v a r i o u s ages, as w e l l as t h e m e d i a n a g e of i n i t i a t i o n . F o r e x a m p l e , in 1991, at a g e 17, 84.0% of h i g h s c h o o l s t u d e n t s h a d d r u n k a l c o h o l , 58.0% h a d s m o k e d a w h o l e c i g a r e t t e , 58.6% h a d h a d s e x u a l i n t e r c o u r s e , a n d 34.3% h a d u s e d m a r i j u a n a . T h e m e d i a n a g e of i n i t i a t i o n of a l c o h o l u s e w a s 14.1 y e a r s .

Alcohol Use

A t e a r l y ages, f e m a l e s t u d e n t s l a g g e d b e h i n d m a l e s t u d e n t s in i n i t i a t i n g a l c o h o l use, b u t b y a g e 15 y e a r s

(1991 s u r v e y ) a n d 17 y e a r s (1993), t h e f e m a l e s h a d e s s e n t i a l l y c a u g h t u p to t h e m a l e s (Table 1). T h e l a r g e s t p r o p o r t i o n a l i n c r e a s e ( o v e r 100% in the 1991 s u r v e y o n l y ) for f e m a l e s t u d e n t s w h o h a d s t a r t e d d r i n k i n g o c c u r r e d b e t w e e n a g e s 13 a n d 15 y e a r s . A m o n g m a l e s t u d e n t s , i n c r e a s e s f r o m a g e s 9 to 11 y e a r s , 11 to 13 y e a r s , a n d 13 to 15 y e a r s w e r e s i m i l a r (65% to 76%). T h e m e d i a n a g e of i n i t i a t i o n w a s a b o u t h a l f a y e a r o l d e r for f e m a l e s t h a n m a l e s in b o t h 1991 a n d 1993. W h e n w e e x a m i n e d the f o u r a g e c o h o r t s (those age 14, 15, 16, a n d 17 y e a r s in 1993), w e f o u n d s o m e significant differences for earlier initiation in y o u n g e r v e r s u s o l d e r c o h o r t s b u t n e v e r o l d e r w,~rsus y o u n g e r (Table 2). F o r e x a m p l e , a m o n g f e m a l e s t u d e n t s curr e n t l y a g e 14 years, 22.7% h a d i n i t i a t e d alcohol u s e at a g e 11 y e a r s c o m p a r e d to 12.1% of f e m a l e s t u d e n t s c u r r e n t l y a g e 17 years. A m o n g m a l e s t u d e n t s , y o u n g e r c o h o r t s also w e r e significantly m o r e likely to initiate alcohol u s e at ages 13 a n d 15 y e a r s t h a n o l d e r cohorts.

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Table 2. Cumulative Proportion of High School Students Who Had Initiated Alcohol Use at Each Exact Age, by Current Age and Gender, 1993 Youth Risk Behavior Survey Current Age (years)

Probability of Initiating Alcohol Use, by Age (years) Females 9 11 13 15 17 Males 9

11 13 15 17

17

16

15

14

Difference

7.4 12.1 21.1 52.2 82.0

(+_1.2) ~ (-+1.4) (_+1.8) (_+2.2) (_+1.8)

8.0 13.2 23.9 57.4

(_+1.2) (_+1.5) (-+2.0) (_+2.2)

10.3 16.5 34.4 69.2

(_+1.5) (-+1.9) (-+2.3) (_+2.3)

10.5 (_+2.1) 22.7 (_+2.8) 42.0 (_+3.5)

13.9 21.8 35.4 62.1 83.0

(_+1.5) (_+1.8) (_+2.1) (_+2.2) (_+1.7)

13.6 23.0 38.0 65.0

(_+1.7) (_+2.0) (_+2.3) (_+2.2)

14.5 25.4 43.3 68.9

(_+1.8) (_+2.2) (_+2.5) (_+2.3)

14.3 (_+2.7) 24.5 (+_3.4) 44.0 (_+3.8)

14 > 15, 16, & 17; 15 > 17" 14 > 15, 16, & 17; 15 > 16 & 17" 15> 16&17;16> 17"

1 4 > 17; 15 > 1 6 & 1 7 " 15 > 17"

a 95% confidence interval. * Significantly different, p K 0.05.

Smoking Patterns of smoking initiation were similar for male students and female students in the 1991 survey, but we found some differences in 1993 (Table 1). For example, in the 1993 YRBS, male students were significantly more likely to report initiation of smoking at ages 9, 11, and 13 years in 1993 than their female counterparts. By age 17 years, about threefifths of both males and females had begun smoking. For both female students and male students, the largest proportional increases in smoking initiation were between ages 9 and 11 years and ages 11 and 13 years. In the 1993 survey, the median age of smoking initiation was 5 months younger for males than females.

In examining the age cohorts (female and male students age 14, 15, 16, and 17 years in 1993), we found no significant differences among the male students, but there were significant differences for earlier initiation in younger versus older cohorts among the female students (Table 3). For example, at age 13 years, 26.8% of female students currently age 14 years had smoked a cigarette compared to 20.8% of female students currently age 17 years.

Sexual Intercourse We found that from age 12 to 16 years, male students were more likely than female students to have had sexual intercourse; at 17 years the proportions were

T a b l e 3. C u m u l a t i v e P r o p o r t i o n of H i g h S c h o o l S t u d e n t s W h o H a d I n i t i a t e d S m o k i n g a W h o l e C i g a r e t t e at E a c h E x a c t A g e , b y C u r r e n t A g e a n d G e n d e r , 1993 Y o u t h R i s k B e h a v i o r S u r v e y Probability of Initiating Smoking a Whole Cigarette by Age (years) Females 9 11 13 15 17 Males 9 11 13 15 17

Current Age (years) 17

16

15

14

3.6 9.1 20.8 42.0 57.6

(-+0.8) a (_+1.2) (_+1.7) (--_2.0) (_+2.1)

3.4 8.4 22.4 41.4

(_+0.8) (+1.2) (_+1.8) (_+2.1)

3.9 10.6 26.8 48.9

(_+0.9) (_+1.4) (_+2.0) (_+2.3)

3.1 (_+1.1) 10.4 (_+1.9) 26.8 (_+2.9)

6.3 15.1 29.3 45.1 60.2

(_+1.0) (_+1.5) (_+1.9) (_+2.1) (_+2.1)

6.8 17.3 32.4 48.0

(_+1.1) (_+1.6) (_+2.0) (_+2.2)

6.3 16.3 29.7 49.0

(_+1.2) (_+1.8) (+2.2) (_+2.4)

7.1 (_+1.8) 13.7 (_+2.7) 31.2 (_+3.4)

a 95% confidence interval. * Significantly different, p K 0.05.

Difference

1 4 > 17; 1 5 > 1 6 & 1 7 " 15 > 16 & 17"

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Table 4. Cumulative Proportion of High School Students Who Had Initiated Sexual Intercourse at Each ]Exact Age, by Current Age and Gender, 1993 Youth Risk Behavior Survey Probability of Initiating Sexual Intercourse, by Age (years) Females 12 13 14 15 16 17 Males 12 13 14 15 16 17

Current Age (years) 17

16

15

14

0.7 2.8 9.2 20.8 37.7 55.4

(±0.4) a (±0.8) (_+1.3) (_+1.7) (±2.0) (_+2.1)

1.4 3.7 9.8 22.8 39.7

(±0.6) (_+1.0) (_+1.4) (_+1.9) (_+2.1)

2.6 5.6 13.2 28.2

(_+0.8) (_+1.2) (_+1.7) (_+2.1)

3.4 (±1.2) 7.0 (_+1.8) 12.7 (±2.5)

6.0 11.2 20.0 29.6 42.3 58.4

(±1.3) (±1.6) (±1.9) (_+2.1) (_+2.2) (_+2.1)

8.0 14.9 23.2 33.6 48.5

(_+1.4) (_+1.8) (±2.1) (_+2.2) (_+2.2)

8.9 15.3 24.8 36.5

(_+1.6) (_+2.0) (_+2.3) (_+2.4)

7.4 (_+2.2) 13.3 (_+3.0) 24.2 (_+3.4)

Difference 14 & 14> 15 > 15 >

15 > 17" 16&1715> 16 & 17" 16 & 17"

15 & 15 > 15> 16 >

16 > 17" 17" 17" 17"

17.

a 95% confidence interval. * Significantly different at p K 0.05.

statistically equal (Table 1). Female students reported the largest percentage increases in first sexual intercourse between the ages of 12 and 13 years, 13 and 14 years, and 14 and 15 years (~100% for each interval); increases for male students were largest between 12 and 13 years and 13 and 14 years. In both surveys, on average, males had initiated intercourse 4 months before females (16.3 vs. 16.6 years). When we examined the age cohorts (1993 ages of 14, 15, 16, and 17 years), we found a number of significant differences among male students and female students in the percentage who had initiated sexual intercourse at various ages (Table 4). In all cases, there were significant differences for earlier initiation of sexual intercourse in younger versus older. For example, at age 14 years, 12.7% of female students currently age 14 years had initiated sexual intercourse compared to 9.2% of female students currently age 17 years.

Marijuana Use Marijuana use differed from the other risk behaviors in that less than half of male students and female students had initiated this behavior by age 17 years (Table 1). More male students than female students began smoking marijuana at early ages, although the differences were not always significant. By age 15 years (1991 survey) and 17 years (1993 survey), the cumulative percentages for female students (having used marijuana) did not differ statistically from those

for male students. The largest proportional increase among female students occurred between ages 13 and 15 years (>200%); male students had proportional increases greater than 100% between ages 9 and 11 years, 11 and 13 years, and 13 and 15 years. In examining the age cohorts, we found no significant differences among the male students in initiation of marijuana smoking at each exact age, but several differences among the female students (Table 5). In all cases, the differences involw.~d a younger cohort having a higher percentage of initiation at a certain age than did an older cohort. For example, at age 15 years, 18.0% of female students currently age 15 years had initiated marijuana use, compared to 12.6% of female students currently age 17 years.

Discussion Using data from two large, nationally representative school-based surveys, we estimated the age of initiation of alcohol use, cigarette smoking, sexual intercourse, and marijuana use for male, and female students in grades 9-12 in the United States. Many female and male students initiated alcohol use, cigarette smoking, and sexual intercourse at very early ages. The median age for female and male students for first use of alcohol was 14 years; for beginning to smoke cigarettes, 15-16 years; and for sexual intercourse, 16-17 years. Extrapolating from our study results, we estimate that among the 3.5

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Table 5. Cumulative Proportion of High School Students Who Had Initiated Marijuana Use at Each Exact Age, by Current Age and Gender, 1993 Youth Risk Behavior Survey Current Age (years)

Probability of Initiating Marijuana Use, by Age (years) Females 9 11 13 15 17 Males 9 11 13 15 17

17

16

15

14

0.8 0.9 3.3 12.6 31.1

(-+0.4) a (_+0.6) (+-0.8) (+_1.6) (_+2.0)

1.1 2.4 5.1 13.5

(+-0.6) (_+0.8) (_+1.0) (_+1.6)

0.7 2.0 4.6 18.0

(_+0.4) (_+0.6) (_+1.0) (_+1.8)

0.3 (_+0.6) 1.0 (_+0.8) 3.8 (_+1.6)

1.3 3.2 10.2 21.9 37.3

(+0.6) (+-1.0) (_+1.4) (_+1.8) (_+2.2)

2.2 4.0 8.6 21.5

(+-0.8) (_+1.0) (+-1.4) (_+2.0)

2.3 3.7 9.8 21.4

(_+0.6) (+-1.0) (_+1.4) (_+2.0)

1.8 (_+1.0) 4.0 (_+1.4) 9.0 (_+2.2)

Difference

16 > 17" 15 > 16 & 17"

a 95% confidence interval. * Significantly different, p < 0.05.

million students who turned 14 years old in 1992, 756,000 females and 748,000 males had initiated alcohol use before their 14th birthday, 482,000 females and 530,000 males had begun to smoke cigarettes, 229,000 females and 411,000 males had had sexual intercourse, and 68,000 females and 153,000 males had smoked marijuana. Female students are less likely than male students to initiate alcohol use, cigarette smoking, sexual intercourse, or marijuana use at very young ages (9 or 11 years for alcohol, cigarettes, and marijuana; 12 or 13 years for sexual intercourse). However, for all of these behaviors, the pace of initiation for female students accelerates so that by age 15 or 17 years, cumulative proportions having initiated each behavior for males and females are equivalent. Using median values for age of initiation, we found a common pattern for both female students and male students; namely that alcohol use started first, followed some 17 or 18 months later by cigarette smoking, then by first sexual intercourse about 10-11 months later. For both female and male students, first alcohol use preceded first sexual intercourse by more than 2 years. Finally, for both female and male students, we found that age of initiation of alcohol use and sexual intercourse may be decreasing. In addition, female students may be smoking cigarettes and using marijuana at earlier ages. Significant differences were found for earlier initiation of the behaviors in younger versus older cohorts. Two factors may be influencing these cohort differences, school dropout and recall bias. The school dropout rate increases rapidly from age 16 years (3.5%) to 19 years (13.5%),

and dropouts have higher rates of risk behaviors than adolescents who stay in school (20). Thus, the probability of having initiated any of the behaviors for the older cohorts may be low owing to school dropout. However, the dropout rate is still only 6.3% at age 17 years (the oldest cohort in t]his study), which suggests that dropouts may account for some but not all of the cohort difference. Other studies have examined recall bias (10,21), which may provide further explanation for these cohort differences. These studies have found that a significant proportion of adolescents, when interviewed at two points in time, reported that they had not told the truth initially, could not remember, or were uncertain about when they initiated the behavior. In studies on age at first sexual intercourse (21), investigators found that the average age of first intercourse crept upward as adolescents got older. Perhaps a similar pattern took place in our study (i.e., older students raised the age at which they first engaged in a risk behavior). Future YRBS data will help clarify whether initiation is occurring earlier or whether reporting biases can explain these findings. Regardless of whether initiation is occurring earlier, these data give cause for concern. We believe that schools should institute intensive intervention programs at least as early as middle or junior high school to help develop the motivation and skills students need to avoid risky behavior. Given that initiation continues on a steady incline at least through age 17 years, schools should provide additional interventions in grades 9-12. Likewise, health care providers should begin screening and counseling regarding health-risk behaviors in early adoles-

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cence. Given the rapid rate at which these behaviors are initiated, an annual adolescent assessment as recommended by the GAPS is needed (22). Some limitations should be considered when interpreting the results of this study. First, the YRBS relies on self-report data. The extent to which missreporting may have taken place cannot be determined. Second, the response categories for the age of initiation variables are grouped, not continuous. The extent to which this may have affected the precision of the estimates also cannot be determined.

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