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JT-T-31 

Antecedents and Correlates of Improved Cognitive Performance 

in Children Exposed in utero to Low Levels of Lead 

David Bellinger 

Alan Leviton 

Jone Sloman 

Children's Hospital (Boston) and Harvard Medical School (D.B., 

A.L.) and Wheelock College (J.S.) 

Running Head: Performance change in lead-exposed children 

Presented at the conference Advances in Lead Research: Implications 

for Environmental Research, National Institute of Environmental 

Health Sciences, Research Triangle Park, NC, January, 1989. 

Address correspondence to : Neuroepidemiology Unit, Gardner House, 

Children's Hospital, 300 Longwood Avenue, Boston MA 02115-5747 

# 

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Abstract 

Up to 2 years of age, children with umbilical cord blood lead levels 

of 10 to 25 ug/dl achieve significantly lower scores on tests of 

cognitive development than do children with lower prenatal 

exposures. By age 5 years, however, they appear to have "recovered" 

from, or at least "compensated" for, this early insult. Change in 

performance between 24 and 57 months of age was examined in. relation 

to level of postnatal lead exposure and various sociodemographic 

factors. Among children with high prenatal lead exposure, greater 

recovery of function was associated with lower blood lead level at 

57 months, higher socioeconomic status, higher H.O.M.E. scores, 

higher maternal IQ, and female gender. The difference between the 

scores at 57 months of children with "optimal" and "less optimal" 

values on these variables generally exceeded 1/2 SD. Higher 

prenatal lead exposure is associated with an increased risk of early 

cognitive deficit. Furthermore, the risk that a deficit will persist 

through the preschool years is increased among children with high 

prenatal exposure and either high postnatal exposure or less optimal 

sociodemographic characteristics. 
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In our prospective study of lead and cognitive development, 

children with "high" umbilical cord blood lead levels (10 to 25 

ug/dl) achieved significantly lower Mental Development Index 

scores through two years of age than did infants whose cord blood 

lead levels were "low" (<3 ug/dl) or "medium" (6-7 ug/dl) (1). 

In contrast to the results of the infant assessments, cord blood 

lead level was not significantly related to children's 

performance on the McCarthy Scales of Children's Abilities at age 

57 months (2). 

These data may be explained in several ways. First, between 

24 and 57 months of age, children "recover" from, or at least 

"compensate" for, the insult represented by high prenatal lead 

exposure. A variant of this is that the extent of "recovery" or 

"compensation" varies among children. Such a contingency might 

be difficult to appreciate using analytical approaches that focus 

only on differences in the mean recovery of groups defined by 

prenatal exposure level. Finding that the likelihood of recovery 

is systematically related to some characteristic or event would 

be particularly important from the standpoint of intervention. 

Alternatively, persisting impact of prenatal exposure is 

obscured by variations among children in postnatal lead exposure. 

Finally, the apparent attenuation of the association between 

prenatal lead exposure and development is a measurement artifact 

attributable to differences in the mix of cognitive skills that 

contribute to a child's scores on the Bayley Scales and the 

McCarthy Scales. For instance, prenatal exposure may be 
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•i 

associated with the skills assessed by the Bayley Scales but not 

with skills assessed by the McCarthy Scales. 

To distinguish among these alternatives, experiences and 

characteristics that correlate with change in performance (both 

improvement and deterioration) need to be identified. In this 

report, we describe our search for these covariates. 

Methods 

Sample 

Of the original cohort of 249 children (3), 204 completed 

the 24 month evaluation. No effort was made to assess at 57 

months 3 children with serious medical problems identified in the 

earlier phase of the study, and 5 sets of twins. Of the 191 

eligible children, we evaluated 170 (89.0%). The median age at 

assessment was 57.8 months. 

Data Collected 

Information on a range of potential covariates of lead 

exposure and cognitive function were obtained (3). These included 

demographics, reproductive history, exposures during and 

characteristics of the index pregnancy, labor and delivery, 

neonatal characteristics, and measures of child rearing 

environment and practices. The outcomes considered here are a 

child's Mental Development Index (MDI) score from the Bayley 

Scales of Infant Development (4) at 24 months and General 

Cognitive Index (GCI) score from the McCarthy Scales of 

Children's Abilities (5) at 57 months. 
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3 

Capillary blood samples were collected when the children 

were 6, 12, 18, and 24 months old and venous samples when they 

were 57 months old. Analytical methods used to measure blood lead 

levels are described elsewhere (6-7). In statistical analyses, 

postnatal blood lead levels were treated as both continuously 

distributed and as categorical variables. The same cut-off values 

used to classify cord blood lead levels as "low" (<3 ug/dl) and 

"high" (>=10 ug/dl) were used to classify postnatal levels. 

Levels between 3 and 10 ug/dl were classified as "medium". 

Data Analysis 

The null hypothesis is that changes in children's 

performance between 24 and 57 months on global tests of cognitive 

development are not related to: 

a) prenatal lead exposure (cord blood lead level), 

b) postnatal lead exposures, or 

c) sociodemographic characteristics (family social class, 

H.O.M.E. score at 57 months, maternal IQ, gender, 

ethnicity, maternal age) 

Using population estimates for the mean and standard 

deviation of MDI and GCI scores (100 and 16, respectively), we 

computed children's z scores for both indices. Additional 

analyses using sample-based estimates of mean and standard 

deviation (115.6 and 16.4 for MDI; 115.5 and 14.5 for GCI) 

produced nearly identical results. As an index of a child's 

"developmental trajectory" between 24 and 57 months, change in z 

score (delta z) was calculated as follows: 
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-'' >»i Pf*f* '— T 

4 

deltas z =» zGCI .5 7 - ZMDI-24 

Thus, a child with a positive delta z improved in performance 

between 24 and 57 months of age. 

Z trans format icons are necessary in order to compare scores 

expressed on different scales. Because MDI and GCI have the same 

expected mean and sitandard deviation, the analyses reported could 

have been conducted) on "delta" scores derived by computing the 

difference between a child's MDI and GCI scores. The more general 

method is used to illustrate its usefulness in identifying 

correlates of performance change and because some additional 

analyses reported involve scores measured on different scales. 

The correlation between delta z and MDI score at 24 months 

was large and negative in the full sample (-0.55) and within each 

strata of cord blood lead (-0.62, -0.42, and -0.58 in the low, 

medium, and high groups, respectively). This association, due at 

least in part to regression to the mean, was not attributable to 

the disproportionate influence of outlying observations. The 

relationship between delta z and MDI at 24 months is linear with 

approximately egual delta z variance for different values of MDI. 

A linear relationship also held within each of the cord blood 

lead strata. Theref fore, in regression analyses of delta z, MDI at 

24 months, a measure of "initial value", was included as a 

predictor. To assess the impact of adjusting delta z for MDI, we 

compared the coefficients assigned to variables in models without 

MDI to coefficients assigned in models that included the MDI 

term. 
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Because of potential problems due to multicollinearity, the 

association between delta z and each variable was adjusted for 

MDI using both a one-stage and a two-stage procedure. In the twostage approach, delta z was regressed on MDI alone and the 

residuals regressed on the variable of interest. In the onestage approach, delta z was regressed on both MDI and the 

variable. The two approaches yielded virtually identical results. 

(Tables are available from the authors). 

Analysis proceeded in three phases: (1) The association 

between delta z and each variable was examined by multiple 

regression analysis in the complete sample. (2) The extent to 

which the association between delta z and a variable differed 

according to cord blood lead stratum was examined using two 

strategies: (a) stratified analyses and (b) fitting regression 

models that included the appropriate interaction term to the data 

for the whole sample. 

Results 

Complete Sample 

Both intrauterine and recent postnatal lead exposures are 

associated with the amount and direction of change in children's 

cognitive performance between ages 24 and 57 months (Table 1; 

column B). The sign of the regression coefficient for cord blood 

lead level (expressed as a continuous variable) is positive, 

meaning that children with higher prenatal exposures tended to 

have higher delta z scores (reflecting greater relative 
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6 

improvement). This is consistent with our previous finding of 

attenuation by 57 months of the association between prenatal 

exposure and cognitive development. 

Using the regression coefficient of .03 assigned to cord 

blood lead level (i.e., the slope of the regression of delta z 

against cord blood lead level) and the mean cord blood lead 

levels of children in the low and high prenatal exposure groups 

(1.8 and 14.6 ug/dl, respectively), the mean changes in 

performance of children in these two groups can be estimated. 

Among low lead children, the mean change was +0.05 standard 

normal deviate units (.03 X 1.8). On an index with a standard 

deviation of 16, this corresponds to +0.9 points (16 X .05). 

Among high lead children, the mean change was +0.44 standard 

normal deviate units (.03 X 14.6) or +7.0 points. Thus, the net 

gain of the high lead children between 24 and 57 months was 6.1 

points (7.0 - 0.9), a substantial percentage of their relative 

deficit (7.8 points) at age 24 months (1). 

The negative signs of the coefficients for blood lead level 

at 18, 24, and 57 months indicate an inverse relationship between 

delta z scores and the children's lead levels at these ages. 

Delta z scores were also significantly related to various 

indicators of sociodemographic status. Children with higher 

H.O.M.E. scores, higher social class, and more intelligent, older 

mothers tended to have higher delta z scores. Delta z was not 

significantly associated with gender or ethnicity. 

The impact of adjusting delta z for MDI score at 24 months 
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is evident by comparing the coefficients in columns A and B of 

Table 1. Those variables significantly associated with delta z 

before adjustment for MDI remained so when MDI was included in 

the model (cord blood lead level, blood lead level at 18 and 24 

months, maternal age). Although adjustment had relatively little 

impact on the magnitude of the coefficients, the standard errors 

were reduced. For several other variables (blood lead at 57 

months, H.O.M.E. score, social class, maternal IQ, and gender), 

adjustment for MDI produced substantial increases in the 

coefficient. Indeed, for all but gender, the coefficient became 

statistically significant. The variable ethnicity, expressed as 

a dichotomy (white/nonwhite), not only had the largest change in 

its coefficient, but 'the sign for whites changed from negative to 

positive. 

Thus, these analyses confirm our previous finding that 

children with high cord blood lead levels showed substantial 

improvement in performance between 24 and 57 months and that 

development in this period was inversely related to postnatal 

lead exposure. In addition, performance change between 24 and 57 

months bore the expected relationships with sociodemographic 

factors, with more optimal values associated with more positive 

change. 

Stratification by Cord Blood Lead Group 

For several variables (blood lead level at 57 months, social 

class, maternal IQ, gender), the coefficient was considerably 
8 

larger among children with high cord blood lead level than among 

children in the other prenatal exposure groups (Table 2). The pvalue for cord blood lead level was considerably more extreme 

among these children than among those in the other groups, but 

this appears to be due mostly to differences in the standard 

errors of the coefficients. The restricted range of prenatal 

exposures among children in the "low" and "medium" cord blood 

lead groups may have contributed to differences across strata in 

the strength of the association between cord blood lead level and 

delta z. 

The association between delta z and blood lead level at 24 

months was comparable in the three cord blood lead groups. This 

reflects in part the fact that in the complete sample, 24 month 

blood lead level was more strongly associated with GCI than was 

any other lead term. In this cohort, performance at 57 months was 

related to level of lead exposure at 24 months, regardless of a 

child's level of prenatal exposure. 

Gender is a particularly striking example of the dependence 

of delta z scores on level of prenatal exposure. The scores of 

girls and boys were almost identical among children with either 

low or medium exposures. Among children with high prenatal 

exposures, however, girls had appreciably higher delta z scores 

(Figure 1). 

Although the associations between delta z and other 

variables appear to depend on initial exposure status (i.e., cord 

blood lead level), interaction terms constructed to assess the 
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9 

significance of these differences were not statistically 

significant. The regression coefficients for many of the 

variables differed significantly from zero only within the high 

cord blood lead group, but these did not necessarily differ 

significantly from the corresponding coefficients in the other 

two cord blood lead strata. For instance, in each stratum, 

children above the median social class tended to have delta z 

scores higher than those of their peers below the median (Figure 

2 ). This social class advantage was greatest, however, among 

children who had high cord blood lead. 

To understand in greater detail the correlates of 

performance change among children with high cord blood lead 

levels, we examined the least-squares mean delta z scores in 

different postnatal lead exposure and sociodemographic strata 

(Table 3). For lead levels at 24 and 57 months and all 

sociodemographic variables, children with more optimal values had 

more positive delta z scores than did children with less optimal 

values. The differences in GCI scores for children in different 

strata can be estimated using the tabled values of delta z. For 

instance, among children who had high cord blood lead but low 

blood lead at 57 months, the mean difference between GCI and MDI 

z scores was +0.45. Among children with high blood lead levels on 

both occasions, the mean difference was -0.16. Therefore, if two 

children with high cord blood lead achieved the same MDI score at 

24 months, but one had a low blood lead level at 57 months while 

the other had a high level, the child with lower exposure would 
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10 

be expected to have a GCI score that is 0.61 standard normal 

deviate units higher (0.45 minus -0.16). Viewed in terms of a 

standard deviation of 16, this corresponds to a difference of 9.8 

points. 

Additional Analyses 

We attempted to assess the extent to which the associations 

between delta z scores and blood lead and sociodemographic 

characteristics reflect differences in the skills measured by MDI 

and GCI scores rather than contingencies that govern the 

persistence and recovery of early lead-associated deficit. To 

make scores at 24 and 57 more closely resemble repeated 

measurements, each child was assigned a "nonverbal" score based 

on his or her performance on a set of 22 items typically 

administered as part of the Bayley Scales at age 24 months (#122-

3, 125, 129, 131, 133-5, 137, 140, 142-3, 147, 151, 153-7, 159-

61). Most of these assess a child's visual-motor coordination and 

integration skills (e.g., ability to copy block structures, speed 

and accuracy in completing formboards, perception of part-whole 

relationships). Sample-based estimates of mean and standard 

deviation were used to transform the total number of items passed 

to a z score. A delta z score was computed by substracting this 

score from a sample-based z score transformation of a child's 

score on the Perceptual-Performance subscale of the McCarthy 

Scales. The set of analyses described above were applied to these 

"delta z: nonverbal" scores. 

Among children with high cord blood lead levels, "optimal" 
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11 

postnatal lead exposure and sociodemographic status were 

associated with higher delta z:nonverbal scores (Table 4). In 

general, the antecedents and correlates of delta z:nonverbal 

scores are very similar to those noted for delta z scores based 

on MDI and GCI (Table 3). Additional tables (analogous to Tables 

1 and 2) are available from the authors. 

i 

These results with delta z:nonverbal scores should not be 

viewed as a definitive test of the hypothesis that the pattern of 

results observed in the main body of analyses is due to 

differences in the mix of skills measured by the Bayley Scales 

and McCarthy Scales. To the extent that we can evaluate this 

hypothesis, however, the evidence suggests that the improvement 

with time is not an artifact of differences in testing 

instruments. 

Discussion 

An association between optimal sociodemographic 

characteristics and reduced likelihood of poor outcome has been 

demonstrated for many early nervous system insults, including low 

birthweight (8-9), prenatal infection (10), failure to thrive 

(11), and Down Syndrome (12). A similar phenomenon has been 

observed with respect to lead exposure (13-15). In previous 

analyses of our cohort, we reported that in the second year of 

life children from lower social classes expressed deficit at 

lower levels of prenatal lead exposure than did children from the 

highest social class (16). 

These observations raise the question of whether children's 
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sociodemographic characteristics are also related to the 

likelihood of "recovery" from a deficit already expressed. The 

data pertaining to this issue are limited to studies showing that 

at-risk infants who receive additional cognitive and/or 

psychosocial stimulation fare better developmentally than do 

children receiving only standard medical care (17-19). 

Through the first two years of life, the children in our 

sample with "high" cord blood lead levels achieved significantly 

lower MDI scores than did children with lower prenatal exposures. 

The association bet-ween high prenatal exposure and lower 

performance persisted beyond age 2, although not as a "main 

effect". Rather, the degree to which deficit persisted varies 

among subgroups of children with different sociodemographic 

characteristics and postnatal lead exposure profiles. The 

associations between "performance trajectory" between ages 24 and 

57 months and several of these characteristics, including high 

social class, high H.O.M.E. score, and high maternal IQ, are 

consistent with the hypothesis that "environmental enrichment" 

facilitates the rate and extent of "recovery" or "compensation". 

The more positive developmental trajectories displayed by girls 

is consistent with previous suggestions that boys are at greater 

risk for many neuropsychiatric adversities (20). Differences 

between children with "optimal" and "suboptimal" covariate values 

tended to be greater among children with high prenatal exposure 

than among children with low or medium prenatal exposure. 

Children already stressed by sociodemographic disadvantage may be 
P.16 

13 

less able to weather the additional stress of "high" prenatal 

lead exposure. 

"Less optimal" is somewhat of a misnomer in our relatively 

advantaged cohort, referring only to values below the median (see 

footnotes b-f in Table 3 for criteria). Most families classified 

as "less optimal" would probably be among the most advantaged 

families in other lead study cohorts (15,21). Our finding that 

children from families with the most optimal sociodemographic 

characteristics "recover" from the insult of high prenatal lead 

exposure may be less relevant to these other cohorts than is our 

finding that children with less optimal values may continue to 

express some degree of developmental deficit. 

The rate of recovery may not be the same in different 

cognitive domains. For instance, following nutritional 

rehabilitation, severely malnourished children show slower gains 

in language than in visual-spatial skills (17). The association 

between family characteristics and a child's response to stress 

is not the same for all outcomes (22). We cannot determine from 

our observations how rate of recovery from early lead exposure 

may differ among cognitive and perceptual functions. 

Summary 

To date, we have observed the following associations between 

lead exposure and development in our cohort: 

1. "High" levels of prenatal lead exposure (>10 ug/dl in cord 

blood) are associated with less rapid cognitive development, at 
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14 

least through 24 months of age (1). 

2. A substantial percentage of children with high prenatal 

exposure contribute to this association (23). 

3. Children below the median in social class express deficit 

at lower levels of prenatal exposure than do children from more 

socioeconomically-advantaged families (16). 

4. At 57 months of age, the mean performance of children with 

high prenatal exposure is indistinguishable from the mean 

performance of children with lower prenatal exposure. Performance 

at this age is, however, inversely related to blood lead level 

measured at age 24 months (2). 

5. Children with high prenatal exposure who also experience 

higher levels of concurrent exposure or who have less favorable 

sociodemographic characteristics (e.g., lower social class, 

H.O.M.E. score, or maternal IQ, male gender) do not improve in 

performance between 24 and 57 months to the same extent as 

children who have comparable levels of prenatal lead exposure but 

are free of these additional adversities. 
'T-,-

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15 

Acknowledgments 

Supported by grants HD08945, HD17407, HD20381, and P30-HD18655 (a 

Mental Retardation Center Grant) from the National Institute of 

Child Health and Human Development, and grant ES00138 (a Research 

Career Development Award to the senior author) from the National 

Institute of Environmental Health Sciences. Other members of our 

research group include Herbert L. Needleman, Michael Rabinowitz, 

Christine Waternaux, and Elizabeth Allred. 
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• V 

is 

References 

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Rabinowitz, M. Longitudinal analyses of prenatal and postnatal 

lead exposure and early cognitive development. New Engl. J. Med. 

316: 1037-1043 (1987). 

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Needleman, H., and Rabinowitz, M. Low-level lead exposure and 

child development: assessment at age 5 of a cohort followed from 

birth. In: Heavy Metals in the Environment, Vol 1 (S. Lindberg 

and T. Hutchinson, Eds.), CEP Consultants Ltd, Edinburgh, 1987, 

pp. 49-53. 

3. Bellinger, D., Needleman, H., Leviton, A., Waternaux, C , 

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387-402 (1984). 

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Neurodevelopment of monitored versus nonmonitored very low birth 

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Total Environ. 40: 45-60 (1984). 

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Berger, 0., Succop, P., and Bier, M. Low-level fetal lead 

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Pediatr 80: 721-730 (1987). 

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Rabinowitz, M. Low-level lead exposure, social class, and infant 

development. Neurobehav. Toxicol. Teratol. in press. 

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18. Grantham-McGregor, S., Schofield, W., and Powell, C. 

Development of severely malnourished children who received 

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(1987). 

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20. Gualtieri, T. Fetal Antigenicity and maternal 

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•-T^awm^mmw**-

19 

of Xead in children. In: Hunan Lead Exposure (H. Needleman, Ed.) 

CRC Press, Inc., Boca Raton, FL, in press. 
20 

Table 1 

Regression coefficients, standard errors (SE) and 

probability values for predictors of delta z in 

models with and without a term for MDI at 24 months 

P.23 

B 

Predictor 

Blood lead 

corda 

6 months 

12 " 

18 " 

24 " 

57 " 

Covariates 

H.O.M.E. 

social cl 

maternal 

maternal 

genderd 

b 

* 

score 

.assc 

IQ 

age 

ethnicitye 

Unadjusted 

for MDI 

coefficient SE 

.04 

-.02 

-.03 

-.22 

-.23 

-.17 

.02 

-.05 

.00 

.03 

.05 

-.41 

.01 

.08 

.08 

.09 

.09 

.12 

.02 

.07 

.00 

.02 

.14 

.31 

P 

.003 

.81 

.75 

.018 

.012 

.16 

.30 

.50 

.72 

.045 

.72 

.18 

Simultaneous 

Adjustment 

for MDI 

coefficient SE 

.03 

.00 

-.08 

-.21 

-.22 

-.23 

.07 

-.22 

.01 

.04 

.18 

.15 

i 

.01 

.07 

.07 

.08 

.08 

.10 

.01 

.06 

.00 

.01 

.12 

.27 

P 

.016 

.96 

.23 

.007 

.005 

.028 

.0001 

.0002 

.003 

.003 

.13 

.57 

a measured cord blood lead level, not category 

b natural log of measured postnatal blood lead levels 

c Hollingshead Two-Factor Index (lower scores represent higher social 

class 

d male coded "0"; female coded "1" 

e nonwhite coded "0"; white coded "1" 
P.24 

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" Tp^ytT- ""UTr^ 

21 

Table 2 

Regression coefficients, standard errors (SE), 

and probability values for predictors of 

delta z in each cord blood lead stratum 

Predictor 

Blood lead 

corda 

6 months*3 

12 " 

18 " 

24 " 

57 " 

Covariates 

H.O.M.E. 

social classc 

maternal IQ 

maternal age 

gender0-

ethnicitye 

coeff 

.04 

-.01 

-.18 

-.13 

-.16 

-.16 

.03 

-.16 

.00 

.04 

-.04 

.04 

low 

se 

.11 

.10 

.09 

.09 

.08 

.14 

.02 

.08 

.00 

.02 

.16 

.27 

P 

.72 

.94 

.063 

.16 

.057 

.26 

.26 

.039 

.42 

.058 

.81 

.88 

Cord Blood Lead 

medium 

coeff 

.10 

.02 

.03 

-.34 

-.31 

-.14 

.13 

-.12 

.01 

.03 

.02 

.37 

se 

.29 

.12 

.13 

.15 

.16 

.22 

.04 

.12 

.01 

.02 

.23 

.60 

Category 

P 

.74 

.88 

.84 

.028 

.065 

.52 

.001 

.29 

.17 

.24 

.94 

.54 

high 

coeff se 

.07 

.01 

-.18 

-.16 

-.28 

-.46 

.07 

-.30 

.02 

.05 

.55 

-.08 

i 

.04 

.13 

.14 

.16 

.16 

.18 

.02 

.10 

.01 

.03 

.22 

.63 

P 

.07 

.95 

.20 

.33 

.079 

.013 

.002 

.004 

.011 

.061 

.017 

.90 

a measured cord blood lead level, not category 

b natural log of measured postnatal blood lead levels 

c Hollingshead Two-Factor Index (lower scores represent higher social 

class 

d male coded "0"; female coded "1" 

e nonwhite coded "0"; white coded "1" 
• V •", 22 

Table 3 

Blood lead 

Means and standard errors (SE) of delta z scores of 

children with high cord blood lead: stratified by 

postnatal lead levels and sociodemographic variables 

Difference in GCI 

scores of low/high 

or optimal/less 

optimal groups 

Postnatal Exposure Group 

low medium high 

57 monthsa 

mean 

SE 

.45 

.21 

14 

14 

-.16 

.20 

9.8 

24 months mean 

SE 

.55 

.21 

05 

,13 

.08 

.23 

10.1 

Covariates 

H.O.M.E.0 

mean 

SE 

social class0 mean 

SE 

maternal IQ^ mean 

SE 

More optimal 

.49 

.14 

.46 

.12 

.39 

.13 

Less optimal 

-.19 

.12 

-.37 

.15 

-.16 

.14 

10.9 

13.2 

8.8 

maternal agee mean 

SE 

25 

14 

.01 

.15 

4.2 

genderJ 

mean 

SE 

34 

14 

-.14 

.15 

7.7 

a natural log of measured postnatal blood lead levels 

b-e "more optimal" defined as values above the median, as follows: 

H.O.M.E.: >52 

social class: class 1 in the Hollingshead Two-Factor Index 

maternal IQ: > 129 

maternal age: >30 

f "female" is the optimal value 
P.26 

ail 

Table 4 

Means and standard errors (SE) of delta z scores of 

children with high cord blood lead: stratified by 

postnatal lead levels and sociodemographic variables 

Blood lead 

57 months0 mean 

SE 

Postnatal Exposure Group 

low medium high 

46 

,27 

35 

18 

-.29 

.27 

Difference in scores 

of low/high or optimal/ 

less optimal groups3 

6.5 

24 months mean 1.04 

SE .26 

02 

16 

-.09 

.30 

9.8 

Sociodemographics More optimal0 Less optimal 

H.O.M.E. 

social 

class 

maternal 

IQ 

maternal 

age 

gender 

mean 

SE 

mean 

SE 

mean 

SE 

mean 

SE 

mean 

SE 

.64 

.18 

.53 

.16 

.42 

.18 

.35 

.18 

.42 

.18 

-.16 

.17 

-.25 

.21 

.00 

.19 

.09 

.20 

-.06 

.19 

7.0 

6.8 

3.7 

2.3 

4.2 

a differences in scores on the Perceptual-Performance subscale of the 

McCarthy Scales of Children's Abilities at age 57 months, computed 

using sample-based estimate of 8.7 as the standard deviation 

b natural log df measured postnatal blood lead levels 

c criteria for "more optimal" scores are the same as those listed in 

footnotes b-f of Table 3 
P.27 

24 

Figure 1: Least squares mean delta z scores for children 

stratified by cord blood lead category and gender. Error bars 

represent one standard error (and for clarity are shown extending 

in one direction only). Means were obtained by modeling delta z 

score as a function of Mental Development Index score at 24 

months of age, cord blood lead category (low, medium, high), 

child gender, and the interaction between cord blood lead 

category and child gender. The difference of 7.7 points between 

the score of boys and girls with high cord blood lead levels was 

obtained by calculating the difference between the mean delta z 

scores of children in the two groups (.48 standard normal deviate 

units) and expressing it in terms of performance on a test with a 

standard deviation of 16. 
25 

P.28 

• • 

CD 

k_ 

O 

ISI C 

•fi l 

CD |s. 

ctj o 

f * •* * 

GJ 

CD 

0.6 r 

0.4 

0.2 

0 

-0.2 

-0.4-

• females 

o males 

7.7 points 

-0.6 

Low Medium High 

Cord Blood Lead Category 
P.29 

fc. , •- / 26 

Figure 2: Least squares mean delta z scores for children 

stratified by cord blood lead category and family social class. 

Error bars represent one standard error (and for clarity are 

shown extending in one direction only). Means were obtained by 

modeling delta z score as a function of Mental Development Index 

score at 24 months of age, cord blood lead category (low, medium, 

high), family social class (above/below median), and the 

interaction between cord blood lead category and social class. 

The difference of 13.3 points between the scores of lower class 

and upper class children with high cord blood lead levels was 

obtained by calculating the difference between the mean delta z 

scores of children in the two groups (0.83 standard normal 

deviate units) and expressing it in terms of performance on a 

test with a standard deviation of 16. 
>>••>' 

P.30 

27 

0.6 r 

O 

%£ 

N c 

^ o ±E 

2 o 

C CM 

CO 

CD 

2 

0.4 

0.2 

0 

-0.2 

-0.4 

-0.6 

• upper social class 

o lower social class 

13.3 points 

Low Medium High 

Cord Blood Lead Category