prepared by Jennifer Ganger
This is a review of the published literature on this subject. The MIT Twins Study will soon have results to add to this question.
I. Introduction
II. The Early Years
III. Progress
IV. Commentary and Further Reading
V. References
There is a pervasive assumption in the twin and language development
literature that twins are somewhat delayed in language development and more
prone to language disabilities.
What I want to present here is a review of conclusions from some
papers I looked through when I was trying to settle the issue of
whether studies on language development in twins are "generalizable"
to the non-twin population. That is, whether or not twins are
significantly different from the non-twin population with respect to
language.
To preview the conclusion, the consensus in the literature seems to be
that being a twin does in fact make a child more prone to language
delays and disorders due to several biological and social factors. As
we shall see, though, any of these factors can also affect singletons;
twins are often just more prone to them. Furthermore, most studies
show twins catching up to their singleton peers on standardized
language tests during early childhood. The conclusion, then, is that
twins should not be considered a special population that is
differentially at risk for the mere fact of being a twin.
Contents
A. Classics
The pervasive assumption of the inferiority of twins in language
originates with two papers from the 1930's which remain two of the
largest studies to date of the language of twins as a group. These
are Day (1932) and Davis (1937).
Day studied 80 pairs of twins and 140 singletons. The children were
aged 1.5 to 5.5. There were 20 pairs of twins at each of 2, 3, 4, and
5 years of age. 50 spontaneous utterances were recorded from each
child while playing with experimenter's toys. On several gross
measures of language complexity (such as sentence length, number of
different grammatical categories in a sentence) twins were found to be
as much as two years behind singletons by the age of five.
Davis carried the same methodology to twins aged 5-9 and found that on
structural measures the twins caught up with singetons on average, but
were still more likely to have articulation problems.
However, there are several flaws in these studies (I am not the first
to point them out). First, no effort was made to exclude twins who
had language, speech, or hearing pathology. Since these are more
common in twins (probably for reasons I'll mention below), it is
possible some of these subjects were included in the sample and
lowered the overall results correspondingly. Second, no information
was reported on birth weight or time of gestation of the twins, both
of which may be factors in language delay. Third, birth order was not
considered as a possible factor, and later birth order is also known
to be associated with language disturbances (Matheny & Bruggeman
1972). Fourth, the twins in this study were not always observed
independently (i.e., separately), and the fact that two children of
the same age were competing for the attention of the experimenter may
have led to shorter sentences. Fifth, the twins were averaged as a
group and then compared to singletons. However, since each twin's
data are not independent of his/her co-twin's, it is not acceptable to
average the results in this way and then compare them to singletons as
a group.
B. Case studies
In addition to these large, (somewhat) controlled studies, there were
a few case studies of twins' secret language, with the assumption that
this inter-twin language was related to the delays experienced with
the culturally transmitted languge (Luria & Yudovich 1959; Zazzo
1960). These are not terribly helpful to the general issue.
III. Progress
Working under the assumption that twins are prone to language delays
and impairments, researchers in the 70's and 80's wanted to know what
factors of being a twin were responsible for the delays. Lytton (1980) and Conway, Lytton, & Pysh (1980) report a study
comparing twins and singletons on language measures. Like the earlier
studies, they also found that twins tended to have shorter sentences
than singletons, less speech overall, less speech directed towards the
mother, and a slight difference in vocabulary as asseseed by an IQ
test. Although there were several biological variables that separated
the two groups (including birth weight, Apgar score, and time of
gestation), environmental variables such as maternal speech to child
were found to account for more of the variance in the language
measures. This suggests that both biological and environmental
variables contribute to language delays in twins, and the authors
conclude that the environmental variables are more important.
Another attempt to sort out these biological variables was presented
by Mittler (1970). He studied 200 twins and 100 singletons at 4 years
of age using the Illinois Test of Psycholinguistic Abilities.
Overall, twins were found to be about 6 months behind singletons at
this age. However, factors other than twin-ness turned out to be
significant predictors of performance. These were birth weight and
age at first word, with those producing their first word later than 18
months performing significantly worse than those producing their first
word before that age.
Another study is reported by Record, McKeown, and Edwards, 1970. They
obtained scores for thousands of twins and singletons on an exam given
in Great Brittain after the 11th year of school. They found that
twins performed about 5 points lower than singletons on this exam on
average. Interestingly, they found that single survivors of twin
pregnancies performed in between the two populations.
Finally Akermann & Thomassen (1991) compared a group of twins and
singletons and found that low birth weight accounted for differences
on standardized language and locomotor tests.
Another relevant study is Hay, Prior, Collett, and Williams 1987.
Although these authors don't examine potential biological factors,
they also document delay in the language development of twins. They
gave 30 month old twins and singletons a test called the Reynell,
which includes a comprehension as well as an expressive component. On
the expressive test, twin boys were found to be 2 to 6 months behind
norms at this age while twin girls were found to be about normal. For
comprehension, twin boys were 0 to 4 months behind, while twin girls
were again at or slightly above normal.
C. Examination of the biological factors alone
An attempt to sort out premature birth and/or low birth weight was
reported by Mohay, Burns, and Luke 1986. They compared low birth
weight twins with low birth weight singletons (less than 1500g). They
used the Griffith Scale of Mental Development which includes a
hearing/speech component. They tested children from one month to four
years of age, and though there were sporadic differences between
groups at some ages and not others, there were no significant language
differences after the age of 2. Furthermore, the overall score
remained within the average range at all ages. And, furthermore, by 4
years, twins were marginally above singletons on several measures.
The implication is that there is some kind of interaction between low
birth weight and twin-ness, such that twins are not affected by it as
harshly as singletons.
In another study, Cescato & Mertin (1986) studied only birth weight
(in singletons) and found that very low birth weight children
continued to lag behind normal birth weight children in the language
subtest of the McCarthy even at the age of 4. So low birth weight may
have detrimental effects, though perhaps more so in singletons than in
twins.
Many researchers have come to the conclusion that it is not biological
but social factors that are responsible for language delays. Several
studies have now found that young twins receive less directed speech
from their caretaker and participate in fewer situations where their
attention is jointly engaged with the caretaker. Both of these
situations are thought to be necessary (to some extent) for language
learning.
A recent summary of twins' language learning situation and how it
differs from that of singletons is quoted below. This is from
"Intelligence, Language, Nature, and Nurture in Young Twins" by
J. Steven Reznick, a professor of psychology at Yale. It appears in
R. J. Sternberg and E. L. Grigoreko (eds), _Intelligence, Heredity,
and Environment_. Cambridge University Press. 1996(?) (Actually I'm
not sure if it's out yet.) Any typos are due to me, and remarks in []
are mine.
Reznick's conclusion is that the twin situation may not be different
from the general sibling situation. The situation he is referring to
is the fact that the lower a child's position in birth order (i.e.,
the later s/he is born) the more disadvantaged s/he is in terms of IQ
scores. In other words, the more older siblings you have, the lower
your IQ. Note that the effect is slight but significant because it
supposedly happens quite consistently, on average. So, there seems to
be a potential disadvantage (in terms of access to environmental
resources) associated with having any siblings, and being a twin is
the most extreme case possible of having a sibling.
I hope this review does not make the situation for twins' language
sound doomed and hopeless. First of all, all the effects mentioned in
this summary are found _on average_, not in every case. Secondly,
environmental and social variables can be influenced by parents in
obvious ways. Furthermore, it is clear that it is not the mere fact
of being a twin that fosters language delay, but rather a combined
effect of perinatal and environmental factors, any of which could
theoretically affect any child, twin or not.
I also recommend chapter 5 (by Kay Mogford) in
for a more thorough review.
I. Introduction
II. The early years
"Twin language is unique in several ways. One aspect of the
'twin situation' is that each twin receives relatively less
individually directed parental speech (Conway, Lytton, & Pysh
1980; Lytton, Conway, & Suave 1977; Stafford 1987; Tomasello,
Mannle, & Kruger 1986). A second aspect is that twins often
participate in three-way conversations in which they may
communicate with either the parent or the co-twin (Savic 1980).
Research on twin versus non-twin language is sparse,
particularly regarding comparisons during the second year for
children speaking English. When quantitative differences
[between twins and non-twins] emerge, they often reflect
auxiliary processes such as a tendency to complete each other's
utterances (Savic 1979) or syntactic or semantic adaptations
to twin status, such as misuse of plurals or pronouns (Malmstrom
& Silva 1986 [note that this is based on one case study].
There is a traditional belief that twin language is developmentally
delayed (Day 1932; Davis 1937; McCarthy 1954), but
this effect may be due to factors not necessarily unique to
the twin situation such as low birth weight or relative birth
position [order] (Matheny & Bruggemann 1972). Moreoever,
detailed analysis of twin language reveals some domains in
which twins are more advanced than singletons. For example,
twins acquire the ability to use "I" relatively quickly
(Savic 1980). Thus the presence of a same-aged sibling
certainly alters the linguistic environment, but twin
language is neither abnormal nor "retarded" . . . Broadly
conceived, the twin situation for learning language is a
special case of the multi-sibling family context, albeit
one in whch there is no space between siblings." [p. 5-6
of pre-publication manuscript]
IV. Commentary and Further Reading
Bishop & Mogford (editors). (1993) Language Development in
Exceptional Circumstances. Lawrence Erlbaum Publishers.
V. References