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Critical periods in childhood learning
Kathy Sylva
Department of Child Development and Primary Education, Institute of Education, London, UK
The human baby is bom with 'hard wiring' that loads the baby to pay attention to
certain things in the environment, especially the communications of canegivers. These
inborn predispositions are gradually shaped by the environment of the family, a
'curriculum for babies' which is rich on communication and making sense of the world.
Day care and nursery education can complement and enhance the child's learning,
especially if they are of high quality. Research has shown again and again that early
learning has lasting effects on development although they are rarely irreversible.
Nature and nurture: an ancient but current debate
This is one of the oldest and most central debates in science. When
discussed by a psychologist it concerns whether a child's development is
governed by a pattern built in at birth or whether it is moulded by
experiences afterwards. Historically the nativist side to the debate, the
side which championed nature, was represented by Plato who believed
that many concepts were innate. On the other side, British 'empiricists'
such as John Locke insisted that the baby's mind is a blank slate at birth
and that all knowledge is etched on it by experience. There is now
widespread agreement that the 'interactionists' have won the day and
that child development is governed by the interaction of nature and
nurture. Still there is heated debate about how much of a child's genes
contribute to the child's eventual intellectual attainment and personal
style. This paper leaves the topic of genes to other authors in the volume
while concentrating on what is known about learning in young children,
especially the power of the environment to shape it.
Inborn bias and constraints
pTic/nth!™ contemporary version of the naturist view focuses on 'inborn
Department of C/11W biases'. These constraints on development do not consist of Plato's
Deve/opmentond inborn concepts but rather ways the newborn is 'programmed' to pay
PnmaryEducation, m o r e attention to some things than to others and to respond in a
Institute or Education, . . . . . „ i n i _ - i L
20 Bedford Way, particular way to certain objects. For example, Slobin1 proposes that
LondonWOHOAL, UK babies are born with what he calls 'operating principles' that
Bnhth Medical Bulletin 1997;53 {No 1).185-197 ©Tti. Bnti»hCounall997
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Fetal and early childhood environment: long-term health implications
Fig. 1 Five models of
the relationship between
maturation and
environment4.The top
model show! a purely
maturational effect' the
bottom model (induction)
shows a purely
environmental effect.The
other three present
interactive combinations:
maintenance, in which
experience prevents
lessening of a
matu rationally
developed skill;
facilitation, in which
experience hastens the
development of some
maturational process;
and attunement, in which
experience increases the
level of a skill or
behaviour above the
'normal' maturational
level (from'1).
Roles of early experience
High-
Low-
High-
~ Low_
| High-
Q.
5o
•o
"o
•5 Low-o
High-
Low-
High-
Low-
Maturation
Maintenance
Facilitation
Attunement
Induction
/' Experience
' No experience
l a )
(b)
(c)
( d )
( e )
Onset of experience
Age
determine which aspects of the auditory environment they selectively
attend to and the systematic ways they try to make sense of speech
sounds. Aspects of the speech environment which babies pay special
attention to are the beginnings and ends to the human speech stream.
It's similar in visual perception; psychologists such as Haith2 claim
that babies have an inborn predisposition to pay special attention to
movement and to shifts between dark and light. Although such biases
in perception are inborn starting points for learning, new skills and
knowledge will be influenced profoundly by experience. However,
these biases constrain the developmental pathways that are possible3;
theories of complete plasticity in human nature have not withstood
scientific test.
186 Brihih M.dico/ Bu//«hn 1997^3 (No. 1)

How nurture modulates nature
Understanding of the interaction between nature and nurture has
become more subtle and powerful. Richard Aslin4 proposes five models
to describe the potential impact on children's development of the
childhood environment in which they grow up (see Fig. 1).
The first model assumes absolutely no environmental effect on the
developing skill: the second assumes that certain environmental support
is necessary for the maintenance of a skill or behaviour but that its form
is innate. In the third model the environment facilitates the earlier or
later appearance of a skill or behaviour which is programmed by the
genes. The last two models show a considerable impact of the
environment. In the fourth model, the environment's shaping leads to
lasting higher or lower performance. For example, it is known that
parents who talk to their children have offspring with higher IQs5.
Finally the fifth model describes skills or behaviours which are shaped
completely by the child's experiences. Exposing a child to a second
language is a good example of the fifth model because the environment
determines all.
The timing of experience: or 'critical periods'
The impact of nurture can vary according to its timing. For example, the
impact of day care on a child may differ according to its occurrence in
the first year of a child's life or the years right before school6. The best
known example of a critical period in animal development is that young
ducks will become imprinted on any moving object in their immediate
environment at approximately 15 h after hatching. If they do not
experience a moving object during this critical period they will fail to
become imprinted at all7.
The broader concept of a sensitive period in human development has
supplanted the notion of critical periods. A sensitive period may last for
months or even years and denotes the time in which the developing child
is particularly responsive to certain forms of experience or particularly
hindered by their absence. A good example is the fact that children in the
period 6-18 months are particularly sensitive to caretaking and that this
is the time when they must develop their core attachment to their
parents8. Other periods may be particularly important for intellectual or
linguistic development, for example the period 12-30 months when
language develops so rapidly9.
Bnfah Mtdical Bulletm 1997,53 (No. 1) 187

The ecology of development
Until recently, most research on child development concentrated on the
child's immediate environment, which was the family throughout
infancy, followed by the school and the peer group. Bronfrenbrenner10
deplores this tunnel vision and argues forcefully that the child's
immediate environment is influenced by broad social factors, such as
cultural beliefs or the distribution of health care. Moreover, even the
environment of the family is not unidimensional; each child inhabits a
unique space which is defined by siblings, the age of their parents when
they were born and a host of other 'ecological factors' which may be
unique to them or their family. (Imagine the difference between the
experiences of the first child born to a Pakistani family who later moved
to Britain and her much younger sibling born 15 years later in a large
city. It would be impossible to imagine that the 'family' influences on
these two children were not vastly different.) An understanding of early
learning and its environment will have to include the social institutions
related to childcare as well as the family.
The 'pre-adapted' newborn
To understand how very young babies learn, we must return to nature
and nurture. Babies' perception of complex patterning provides a good
example of how the infant's 'wiring' influences what it learns about the
visual world. Newborns and infants have far more sensory capacity than
doctors or psychologists have suspected. Babies' motor skills are limited
for many months and perhaps professionals assumed that sensory skills
were as well. Although the newborn baby does not have the sensory
capacities of a 6 month old, most of the basic perceptual skills are
functioning at some level immediately after birth. The newborn is much
better at getting information about the world than at acting on it.
Although newborns cannot judge depth at birth and are clumsy at
reaching, their behaviour is governed immediately by visual information
which they process quite efficiently. Babies look at the world in a
systematic way. Haith2 says 'there are rules babies look by' and adds that
the rules change with age. At first, visual attention is focused on where
objects are but a change takes place around 2 months such that attention
is drawn more to the characteristics of objects rather than their
location11. The baby moves from a visual strategy for 'finding where
things are' to one of 'finding out about things'. It's not known whether
this shift is affected by learning but it is certainly an inborn disposition,
18 8 British Medical BuH.hn 1997,53 (No 1)

since babies reared in vastly different environments all go through the
same perceptual shift.
Most interesting of all, learning concerns the baby's recognition of the
parents. Walton et at11, showed newborns videos of their own mothers
contrasted with videos of a woman with similar hair colour, eyes,
complexion and hair style. Babies of 1-2 days clearly recognised their
own mothers because they spent significantly more time looking at them.
Within 2 days, the baby had learned the perceptual features of his/her
own mother's face, a sure example of learning shaped by inborn rules
concerning what to pay attention to.
Developmental changes in thinking and learning
Jean Piaget13"15 was the first developmental scientist to take seriously the
interaction of nature and nurture in children's thinking. He began by
studying what he called 'sensory-motor schemas' in infancy. These were
part of the baby's biological inheritance and included looking, grasping
and sucking. Through careful study of his own children, Piaget
concluded that babies did not take in information in a passive way, as
a camera might do, but they use environmental information gathered
from inborn actions such as looking and sucking to construct a model of
the world. For Piaget, learning is not the passive acquisition of
information; instead, babies and children actively process information
entering through the senses and integrate it into representations of
previous information. This means that children do not receive
information; they process it in light of what they already know and
they integrate it with previous knowledge.
Piaget stressed the fact that children are much more than miniature
adults with fewer facts at their command. Children perceive and think
about the world differently, according to developmental stages. Piaget
investigated the regular sequences children follow as they develop
concepts. Children all over the world pass cognitive milestones at the
same age and even make certain mistakes at the same time. For example,
when children learning English discover the '-ed' rule for the past tense,
they start to make mistakes they haven't made before. A 3 year old who
had been regularly heard to say 'I rode my bike' suddenly changed to 'I
rided', incorrectly generalising a new-found rule to irregular verbs.
Although grammar varies from language to language, the tendency to
over-generalise morphological endings appears in all cultures, showing
again an inborn predisposition to pay attention to beginnings and
endings in language.
Bnhi/i Mtdical Bulletin 1997^3 (No 1) 189

Piaget's most important notion is that the child is an active participant
in the development of knowledge and constructs his/her own under-standing.
At each stage in development, the child adapts to the world by
using different sorts of mental operations.
Piaget's stages of mental development in young
children
Sensory-motor stage
During this stage, the infant responds to the world through sensory and
motor schemes. The infant lives in the here and now, having little
concept of the future, and does not plan. Piaget argued that the child in
the period 0-2 years had simple internal representations of the outside
world. The child lives in and acts on the world but does not reflect on it.
Although recent research suggests that babies can be planful in searching
for things and that they may know more about objects than Piaget
originally thought, most psychologists agree that thinking in the pre-verbal
child is oriented to the here and now, especially to the child's own
view of things.
Pre-operational stage
During the years 2-6, Piaget claimed that language lifted children's
thinking to a new level. In this stage, children begin to pretend, to plan
events in the future and to interrogate their own past experiences in a
systematic way. Piaget described the thinking of children in this stage
mostly by things they could not yet do: they were not good at taking the
points of view of others; they were not good at classifying things into
hierarchical groupings, and they tended to understand or measure things
according to appearances rather than underlying mathematical
principles.
Concrete operations stage
At about 6 years of age, children abandon their egocentric and non-logical
thought as they develop an organised system of scientific and
mathematical concepts.
190 Bnhih Mtdical Bulletin 1997,53 (No. 1)

The lasting impact of early learning
We turn to the evidence which supports the widely held belief that early
learning in the stages just described has lasting impact. Early learning is
considered first inside the home and next as it occurs in day care or
nursery school.
How the family environment shapes children's learning
Hundreds of studies have explored the relationship between social
background and children's intellectual attainment. Broman et alu,
studied more than 50,000 children born in the US and found, not
surprisingly, that the average IQ of children rose with both social class
and maternal education. British studies show the same17-18. The more
interesting questions concern just how certain parents and families
create a stimulating and empowering environment. (The influence of
genes on this topic is discussed elsewhere.)
Broman et at16 showed clearly that the powerful influence of maternal
education on children's IQ can be detected after taking into account
social class; in other words, the mothers with more education had
children with higher IQs within each social class. How do parents help
or hinder their children's learning? Bee19 summarises five dimensions of
family interaction and stimulation which make a difference to children's
learning. Researchers have found that parents of children with high IQ's
seem to do the following:
1. They provide the child with an interesting and complex physical
environment which includes toys appropriate for the child's
developmental level20'21.
2. They are emotionally responsive to the child and involved in their
daily lives. This can be seen in their warm, contingent reactions to the
child. They smile when the child smiles, talk when the child talks and
answer the child's questions22"24.
3. They talk to their child, using language that is accurate and richly
descriptive5.
4. They are not excessively restrictive or punitive. Instead they give the
child room to explore and even to make mistakes21*25.
5. They have high expectations for their child's learning and emphasise
achievement, especially educational achievement26.
Two decades ago, researchers discovered the 'structural' influences on
children's learning and attainment, e.g. social class, ethnicity or maternal
Bntit/i MtdKal Bvlhttn 1997;53 (No 1) 191

T)rp« of tejt
IQ
— — Readiness
Achievement
- 2 0
Immediate 1st Year 2nd Year 3+ Years
Fig. 2 Immediate effects and long-term effects of Head Start on IQ, school readiness and achievement meosures (treatment control
studies). From McKey alaP7.
education. Now they have developed techniques involving fine grained
analysis to specify the activities and objects inside the home which seem
to make a difference.
Research on learning in preschool settings
The American project, Head Start, has received government funding for
two decades in hopes that it would 'break the cycle of poverty'. Initial
evaluations seriously underestimated the value of the programme by
focusing on measures of intelligence as the main outcome. Sadly they
found that early IQ gains quickly washed out, leaving graduates of Head
Start no different from control children.
Recent evaluations have employed sophisticated research methods to
look at a wide array of child outcomes from early education. In 1985, a
synthesis of research findings was published27 which combined into a
single meta-analysis the results of 210 studies evaluating the impact of
Head Start. To enable comparison amongst the studies, findings were
converted to statistical 'effect sizes' and comparisons were made across
different sites and target groups.
McKey and his colleagues concluded that Head Start had immediate,
positive effects on children's cognitive ability27. Unfortunately, the
192 Bnfish Medical Buffctm 1997,53 (No 1)

cognitive gains were no longer apparent after the end of the second year
at school (see Fig. 2). Head Start also had short-term positive effects on
children's self-esteem, scholastic achievement, motivation and social
behaviour, but these advantages also tended to disappear by the end of
the third year in school.
The smaller, better controlled studies of the effects of Head Start have
produced more positive findings. A well designed study by Lee et al28
compared the outcomes of 969 disadvantaged children who had
experienced three different pre-school environments; Head Start, some
other pre-school programme and no pre-school. Large, initial differences
on a wide range of outcomes were found at school entry, with Head
Start children lower on almost all measures. After adjusting for initial
scores (because the Head Start sample were lower), Head Start children
showed larger gains on measures of social and cognitive functioning
('readiness for school'20) compared to children in the other two groups.
It was not surprising that children in Head Start began school with lower
scores because Head Start children tend to be drawn from families of
low levels of income and education. Thus, in Lee's study28, Head Start
was effective in 'closing the gap' but did not succeed in doing so
completely because its children began at greater levels of disadvantage.
Notable in Lee's study28, were the large gains made by black children
in Head Start. In many evaluative studies of pre-school it has been
shown that pre-school intervention is particularly effective for the most
economically disadvantaged children29. Black children gained more than
white children, even when controlling for initial levels of ability. Further,
black students of below average ability gained more than their
counterparts of average ability. They concluded that their study
demonstrates the effectiveness of Head Start: 'not only were those
students most in need of pre-school experience likely to be in Head Start
programs, but also that those black students who exhibited the greatest
cognitive disadvantage at the outset appeared to benefit most from Head
Start participation' (p. 219).
There is cause for optimism when examining research on the
effectiveness of pre-school programmes which are of 'high quality'. A
group of American researchers carried out a meta-analysis of the effects
of compensatory education on well resourced, 'quality' programmes.
They ignored the garden-variety programmes (which included Head
Start) and focused instead on projects of excellent curricular quality and
rigorous research design. Lazar et al30 restricted their analysis to 11
carefully monitored programmes, using a statistical analysis enabling
researchers to compare effect sizes across many different studies. The
researchers located approximately 2000 pre-school 'graduates' and their
matched controls to describe their educational and employment
histories. In addition they interviewed the youth and their families.
Bnttth Medical BuHef.n 1997,53 (No 1) 193

Results from the 11 studies showed that attendance at excellent,
cognitively oriented pre-school programmes was associated with later
school competence at the age of 19 and reduced likelihood of assignment
to 'special' education. Interviews carried out at age 19 showed the
families of the nursery group to have higher aspirations for their
children's employment.
The most carefully controlled of the 11 programmes reviewed by
Lazar was the Perry Pre-school Project, which became known later as
High/Scope. This curriculum is of exceptionally high quality, and it
includes a complex training scheme for staff and sound parent
participation. The programme has been subjected to careful evaluation
for almost 30 years and has consistently shown striking results.
Although an initial IQ advantage for pre-school graduates disappeared
by secondary school, there were startling differences in outcome between
the 65 children who attended the half-day educational programme over
2 years and the control group of 58 children who had remained at home.
Figure 3 summarises the findings at age 27 when the High/Scope
'graduates' had:
• significantly higher monthly earnings at age 27 (29% vs 7% earning
$2,000 or more per month)
• significantly higher percentage of home ownership (36% vs 13%) and
second car ownership (30% vs 13%)
• a significantly higher level of schooling completed (71% vs 54%
completing 12th grade or higher)
• a significantly lower percentage receiving social services at some time
in the past 10 years (59% vs 80%)
• significantly fewer arrests by age 27 (7% vs 35% with 5 or more),
including significantly fewer arrested for crimes of drug taking or
dealing (7% vs 25%)
Schweinhart and Weikart31 carried out a cost-benefit analysis which
showed that for every $1000 that was invested in the pre-school
programme, at least $7160 (after adjustment for inflation) was returned
to society. These calculations were based on the financial cost to society
of juvenile delinquency, remedial education, income support, and
joblessness — set against the running costs of an excellent pre-school
programme.
The effects of day care on children's outcomes
Most research studies have looked at the effects of day care on children's
emotional adjustment, especially their attachment to their mothers32-33.
194 Bnhsh AWica/ Bulletin 1997,53 (No 1)

Fig. 3 High/Scope
Perry Pre-ichool Study,
from Schweinhart and
Weikart31.
High/Scope Perry Preschool Study:
Outcomes at age 27
High School Grades
Five or more arrests
$2000+ Eamings/Mtri
Soc Serv since 18
Own a House
20 40 60
Percentage
I Programme
• No programme
80 100
Several American scholars claim that early entrance into day care, before
the age of 1 year, is detrimental to children's emotional development6,
but others claim that early entry to day care will not harm children
whose parental attachments are secure34. The debate rages on.
Research from Sweden tells a very different story; Andersson35 found
day care experience gave children a better start in school. He examined
the development of 128 children who attended well resourced
neighbourhood day care centres. Progress was monitored from the
children's first year in day care to the age of 13. No developmental
disadvantage was found in the day care group compared to children who
had stayed at home. In fact, the highest performance in school tests and
the best emotional adjustment was found in the children who had
experienced the MOST day care, even before the age of 1 year.
Why do Swedish children appear to benefit from attendance at child
care centres when some American studies suggest that day care
attendance may lead to poor social and emotional adjustment? Perhaps
the answer lies in different social policies, with Sweden offering highly
subsidised day care to families from all walks of life and the US offering
private lower-quality care.
Howe36 studied 80 children in deliberately contrasting care in the US.
Half were enrolled in excellent centres and half in poor ones. 'High
quality' centres were characterised by the following: (i) stable child care
arrangements such that children interacted with just a few primary
caregivers in any one day; (ii) low staff turnover so that children were
cared for by the same individuals over several years; (iii) good staff
training in child development; and (iv) low staff:child ratios, e.g. from
0-12 months the ratio was 1:3, from 1-3 years the ratio was 1:4, and
from 4-6 years the ratio was 1:8-12.
Bnfufc AWica/BuH.hn 1997,53 (No 1) 195

In Howe's research, outcomes were controlled for family background
and individual differences, factors that might affect development as well
as the quality of care36. After controlling for these, children enrolled in
the higher quality centres still did better in primary school on both
educational and social measures. The picture was different in the low
quality centres, with children doing particularly poorly at school when
they had been enrolled in lower quality centres before their first
birthdays when they entered primary school. These 'early entry' children
were distractible, low in task orientation and had considerable difficulty
getting on with peers.
Research in both the US and Sweden shows clearly that day care,
when of high quality, does not harm children's development and may
enhance children's learning. There have now been many studies
confirming the fact that the higher the quality of day care, the better
the learning outcomes for children37"40.
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