 42. Some Contributions of Economics to the Study of Personality 877
Evidence from the Perry Preschool Program 
and Other  Interventions
Evidence from the Perry Preschool Pro -
gram shows how noncognitive skills can 
be changed in ways that produce beneficial 
lifetime outcomes. The Perry Preschool Pro -
gram enriched the lives of 3- and 4-year-old 
low- income Black children with initial IQs 
below 85 at age 3.74
Participants were taught social skills in a 
“plan-do-  review” sequence where students 
planned a task, executed it, and then re -
viewed it with teachers and fellow students. 
They learned to work with others when 
problems arose.75 In addition, home visits 
promoted parent–  child interactions. The 
program ended after two years of enrollment 
and both treatments and controls entered the 
same school. The program was evaluated by 
the method of random assignment.
The program did not improve IQ scores 
in a lasting way. Figure 42.13 shows that, 
by age 10, treatment and control groups had 
the same average IQ scores. Many critics 
of early childhood programs seize on this 
finding and related evidence to dismiss the 
value of early intervention studies. Similar 
evidence from Head Start programs and a 
faith in IQ as a central determinant of life 
success strongly influenced Arthur Jensen’s 
(1969) views about the genetic determina -
tion of skills.Nevertheless, the program improved out -
comes for both boys and girls, resulting in 
a statistically significant rate of return of 
around 6–10% per annum for both boys and 
girls (see Heckman, Moon, Pinto, Savelyev, 
& Yavitz, 2010). These returns are above 
the post–World War II, pre-2008 meltdown 
in stock market returns to equity estimated 
to be 5.8% per annum.76
The Perry Preschool Program worked pri -
marily through improving personality skills. 
Participants had better direct measures of 
personal behavior (a weighted average of 
“absences and truancies,” “lying and cheat -
ing,” “stealing,” and “swears or uses ob -
scene words” measured by teachers in the 
elementary school years). Participants of 
both genders improved their “externalizing 
behavior,” a psychological construct related 
to agreeableness and conscientiousness. For 
girls, the program improved openness to ex -
perience (proxied by academic motivation). 
The program also improved scores on the 
California Achievement Test (CAT). This 
supports the previous evidence that shows 
performance on achievement tests depends 
in part on personality skills. Arthur Jensen’s 
lifetime campaign against early intervention 
program was based on using faulty measures 
of relevant lifetime skills.
Other studies are broadly consistent 
with the evidence from the Perry Preschool 
study. Analyses of data from Project STAR 
FIGURE 42.13.  Perry Preschool Program: IQ by age and treatment group. Note: IQ measured on the 
Stanford–  Binet Intelligence Scale (Terman & Merrill, 1960). The test was administered at program 
entry and at each of the ages indicated. From Cunha, Heckman, Lochner, and Masterov (2006) and 
Heckman and Masterov (2007), based on data provided by the HighScope Foundation.100IQ95
90
85
80
75
4 Entry78.583.3 83.586.387.1 86.9 86.8
84.68587.7 88.191.7 91.394.995.5
5 6
AgeTreatment group
Control group
789 1079.6