Imaginary playmates have fascinated psychologists, parents, and teachers for many years. Although psychologists have been writing about imaginary playmates since the late 1800s, only a handful of articles and book chapters exist on this topic, with only a few of those empirically based. Some experts think that children with imaginary playmates are likely to be between the ages of three and six, be of at least average intelligence, possess good verbal skills, be characterized as creative and cooperative with adults, and be an only child. They also tend to come from families that value active rather than passive behavior and who watch less television than their peers. Imaginary playmates are drawn from television, stories, or real people, or can also be original characters developed by the child.
Having an imaginary playmate is typically assumed to have a positive effect on children's social and cognitive development. Contributions to social development are thought to include increased opportunities for practicing positive social skills, taking another's perspective, and experimenting with relationships. The assumed cognitive benefits associated with having an imaginary playmate include the ability to engage in creative and original thought, as well as to use abstract reasoning skills. In a 1992 arti cle, however, S. Harter and Christine Chao reported that children with imaginary playmates were rated as less competent in cognitive, physical, and social skills than their peers who did not have imaginary playmates, though the researchers cautioned that these findings had to be replicated before they could be viewed with confidence.
There are significant differences in reported prevalence rates. Older studies found that about 15 percent to 30 percent of preschool children had an imaginary friend, whereas Dorothy Singer and Jerome Singer found in 1990 that 65 percent of the young children had an imaginary playmate.
Regarding gender differences, in one study boys tended to have imaginary friends who were more competent than they were and girls tended to have imaginary friends that were less competent (Harter and Chao 1992). Another study found that while the majority of both boys and girls had same-sex imaginary friends, more girls than boys had friends of the opposite gender, and boys had more nonhuman imaginary friends than girls did (Manosevitz, Prentice, and Wilson 1973).
Clearly more research is needed in order to understand the characteristics of the children who develop an imaginary playmate, the benefits associated with having an imaginary playmate both long- and short-term, and the role of adults in supporting social and cognitive development through interactions related to the imaginary playmate.
See also: FRIENDSHIP
Gilbertson, S. A. ''Play Behavior in Preschool Children: Relations to Imaginary Companions.'' Paper presented at the meeting of the Rocky Mountain Psychological Association, Denver, CO, 1981.
Harter, S., and Christine Chao. ''The Role of Competence in Children's Creation of Imaginary Friends.'' Merrill-Palmer Quarterly 38 (1992):350-363. Hurlock, E. B., and W. Burstein. ''The Imaginary Playmate.'' Journal of Genetic Psychology (1932):390-392. Manosevitz, Martin, Norman M. Prentice, and Frances Wilson. ''Individual and Family Correlates of Imaginary Companions in Preschool Children.'' Developmental Psychology 8 (1973):72-79.
Singer, Dorothy G., and Jerome L. Singer. ''Imaginary Playmates and Imaginary Worlds.'' In The House of Make-Believe. Cambridge, MA: Harvard University Press, 1990. Somers, Jana U., and Thomas D. Yawkey. ''Imaginary Play Companions: Contributions of Creative and Intellectual Abilities of Young Children.'' Journal of Creative Behavior 181 (1984):77-89.
Svendsen, Margaret. ''Children's Imaginary Companions.'' Archives of Neurological Psychology 32 (1934):985-999.
Vostrosky, C. "A Study of Imaginary Play Companions." Education 15 (1895):383-397.
Rebecca B. McCathren M. Gutierrez G. Holliday
Immunization is recognized as one of the greatest public health achievements of the twentieth century. The widespread use of immunization is responsible for dramatic reductions in, and in some cases the elimination of, specific infectious diseases.
Immunizations can partially or completely prevent illness by a specific microorganism. By preventing illness, immunizations avert the acute effects of disease, complications of disease, and long-term disability related to disease. When immunizations are widely used, the spread of disease within the population can also be prevented. By preventing outbreaks of disease, immunizations reduce health-care expenditures, including the costs of: (1) prescription and over-the-counter medications, (2) health-care provider visits (including office and emergency room visits), (3) hospitalization, and (4) long-term disability or long-term care. Immunizations also save money by reducing the number of days of work loss by employees because of personal illness or illness in a dependent family member.
Immunizations can provide active or passive protection against an infectious disease. In active immunization, entire organisms (e.g., inactivated bacteria; live, weakened virus) or their parts (e.g., bacterial tox-oid; inactivated, viral antigen) are administered. The immune system responds to the vaccine by producing a long-lasting, protective immune response in the recipient. Examples of active immunization include all of the vaccines used in the standard childhood immunization schedule (see Table 1). In passive immunization, preformed antibodies against specific microorganisms are administered. Protection lasts only months because of the relatively short half-life of the antibodies. Passive immunization is used before or immediately after an exposure to an infectious agent to prevent infection. Passive immunization is used for a number of infectious agents, including hepatitis B, rabies, respiratory syncitial virus, tetanus, and varicella-zoster. The remainder of this discussion will be directed toward active immunizations used during childhood.
Immunizations can be recommended on either a universal or a selective basis. Universal immuniza tions are directed at all members of a population. The eventual goal of immunizing all susceptible individuals is the complete eradication of a disease, as in the case of smallpox. Selective immunizations are directed at individuals who are considered at high risk of a disease, or at high risk of complications of a disease.
In the United States, the choice and timing of immunizations are made jointly by three national organizations—the Advisory Committee on Immunization Practices branch of the federal government's Centers for Disease Control and Prevention, the Committee on Infectious Diseases of the American Academy of Pediatrics, and the American Academy of Family Physicians. The complete schedule for universal immunizations is updated and published annually (see Table 1). Alterations to the schedule (e.g., addition of newly approved vaccines and changes in the timing of vaccines) can nevertheless be made throughout the year.
Immunization against smallpox is an example of the success possible through universal vaccination programs. Accounts of immunization against smallpox were reported as long ago as the 1600s. At that time, uninfected individuals were exposed to material (e.g., pus) from patients suffering from mild disease in the hopes of preventing more serious or fatal disease. In the late 1700s, Edward Jenner, a physician in England, promoted the widespread use of the cowpox virus to prevent smallpox. Two centuries later, on October 26, 1979, the World Health Organization declared that smallpox had been eradicated from the entire world.
The success of other immunization campaigns in the United States is shown in Table 2. The incidence of vaccine-preventable disease has been reduced between 97.6 and 100 percent through the use of universal immunization.
This success could not have been achieved without the combined efforts of researchers, health-care providers, and families. Researchers are responsible for the development of a wide variety of safe, effective vaccines against common childhood diseases. Healthcare providers are responsible for ensuring that children receive the appropriate and required immunizations. And families are responsible for bringing their children in for routine child-health supervision visits. At the beginning of the twenty-first century in the United States, record numbers of children were being immunized. This is an essential part of the success of immunizations in this country.
A combination vaccine against diphtheria, tetanus, and pertussis (DTP) was first licensed in the 1940s. The initial vaccine consisted of diphtheria and tetanus toxoids (a weakened form of the toxin that actually does the damage in the infections), and inactivated, whole pertussis bacteria. As seen in Table 2, tremendous reductions have been achieved in all three diseases. Use of the original whole-cell pertussis vaccine was marred in the past by concerns that the vaccine could cause brain injury (specifically, an en-cephalopathy). While this was largely disproven, the concerns were enough to lead many people to refuse the vaccine in the 1970s. In both Great Britain and Japan, the decline in immunization coverage resulted in epidemics of pertussis. In Great Britain alone, more than 100,000 cases of pertussis occurred between 1977 and 1979. In both countries, vaccination programs were restarted after the consequences of low immunization rates were seen. In the United States, a switch from the whole-cell pertussis vaccine to an acellular preparation with significantly less fever and local reactions was made in 1991.
The first polio vaccine was the injectable, inactivated polio vaccine (IPV) introduced in 1955. The live-attenuated oral polio vaccine (OPV) was licensed in 1960. Since the polio virus attacks the nerve cells that control muscle movement, vaccines against polio are responsible for enormous reductions in paralytic poliomyelitis throughout the world, and for the eradication of natural polio infection from the entire western hemisphere. In the United States, the OPV was used principally from the 1960s until 1997. Since 1997, a transition has been made to the IPV in order to eliminate any chance of vaccine-associated paralytic poliomyelitis caused by OPV. IPV has no risk of
Declines in Vaccine-Preventable Childhood Diseases in the United States
Disease Maximum Year 1998 Percent
# ol Cases Change
Diphtheria 206.939 (1921) 1 99.9-
Haemophilus inlluenza 20.000' (pre-1985) 54 99.7-
Measles 894.134 (1941) 89 99.9-
Mumps 152,209 (1968) 606 99.6-
Pertussis 265.269 (1934) 6.279 97.6-
Poiiomyelitis (paralytic) 21.269 (1952) 0" 100.0-
Congenital Rubella 20.000* (1964-1965) 5 -99.9
Smallpox 48.164 (1900-1904) 0 100.0-
' "Excludes one case ol vaccine-associated polio reported in 1998
SOURCE: Alan Uba.
causing paralytic poliomyelitis. A complete switch to IPV occurred in the United States in January 2000.
The first live, attenuated measles vaccine was licensed in 1963, followed by mumps and rubella (German measles) vaccines in 1967 and 1969. The combined measles, mumps, and rubella (MMR) vaccine has been available since 1971. In the 1980s, epidemics of measles in the United States demonstrated the importance of immunizing and reimmunizing against measles. Concerns have been expressed over a possible link between autism and the measles vaccine, and this issue is discussed below under ''Controversy over Vaccination.''
The first hepatitis B vaccine was licensed in 1981. In the United States, an attempt at selective immunization of individuals (e.g., those having contact with blood or blood products, including health-care workers) with the hepatitis B vaccine did not control the number of new cases. Universal immunization of infants against hepatitis B with a vaccine began in 1990.
The Haemophilus influenzae type b (HIB) vaccine was first licensed in 1985. The initial vaccine could only be used in older children because it did not evoke protective immunity in younger infants. Unfortunately, most HIB disease occurs in the first two years of life. Subsequently, a new vaccine was introduced in 1990 that proved extremely effective in early infancy. By 1998, rates of serious bacterial infection due to HIB had declined by 99.7 percent since the introduction of the newer HIB vaccine in 1985.
A chicken pox (varicella) vaccine was licensed in 1995. Before the vaccine was available, an estimated four million cases of chicken pox infection occurred in the United States each year. While most cases of natural infection were uncomplicated, chicken pox was responsible for an estimated eleven thousand hospitalizations and one hundred deaths per year. Once the vaccine was available, chicken pox became the most common vaccine-preventable cause of death in the United States. Universal immunization with the chicken pox vaccine began the same year that the vaccine was licensed.
The pneumococcal-conjugate vaccine was licensed in 2000. Streptococcus pneumoniae (pneumococ-cus) is a leading cause of serious bacterial infection in childhood, including pneumonia, bacteremia (bacteria in the blood), and meningitis. Pneumococcus is also the most common cause of ear infections in children.
Selected immunizations are directed at high-risk populations. These populations include: (1) individuals with underlying immune system disorders, (2) individuals with chronic underlying medical conditions that make them more susceptible to severe infection, and (3) individuals with increased risk of contracting infection.
The success of universal immunization campaigns requires high rates of immunization. Factors that interfere with the delivery of immunizations in clude: (1) lack of access to health care, (2) lack of knowledge about appropriate immunizations for children, (3) misconceptions about contraindications to vaccination (reasons that vaccination may be inadvisable), and (4) missed opportunities for immunizations.
Public fears about the possibility of adverse central nervous system effects of immunizations have followed several routine childhood vaccines. In the 1970s there were concerns over neurologic side effects (primarily, encephalopathy) of pertussis vaccination. In the mid-1990s there were concerns over central nervous system demyelinating disease (e.g., Guillain-Barre syndrome, multiple sclerosis) and the hepatitis B vaccine. While concerns over pertussis and hepatitis B vaccines have largely diminished in the United States, fear increased in the late 1990s over a possible association between autism and the MMR vaccine.
Controversy over the MMR vaccine followed publication of an article in the journal Lancet in early 1998 written by Andrew Wakefield. Based on observations and investigations made in twelve children, the authors suggested a link among the MMR vaccine, chronic intestinal inflammation, and autism.
Two subsequent epidemiologic studies, by B. Taylor and L. Dales, failed to identify an association between the MMR vaccine and autism. The Taylor study, from the United Kingdom, demonstrated increasing rates of autism, but a comparison of rates before and after the MMR vaccine was introduced in the United Kingdom in 1988 failed to uncover a link between the two. The Dales study, from California, demonstrated an almost fourfold relative increase (373%) in autism between 1980 and 1994. Immunization rates, however, increased by only 14 percent during the same period.
Until the cause or causes of autism are better defined, controversy will continue in this area. Currently, there is little, if any, scientific evidence linking the MMR vaccine and autism. Meanwhile, the global eradication of measles is still a possibility through widespread use of measles-containing vaccines. Eradication of measles would eliminate the estimated 880,000 deaths that occur worldwide as a result of measles infection.
The immunization schedule is constantly evolving. Future changes include vaccines against additional diseases, new vaccine combinations, and novel approaches to immunization. New routes for vaccine administration (e.g., nasal vaccines, vaccines incorporated into foods) are also being evaluated.
See also: BIRTH DEFECTS; RUBELLA
American Academy of Pediatrics. Committee on Infectious Diseases. Red Book, 2000: Report of the Committee on Infectious Diseases, 25th edition. Elk Grove Village, IL: American Academy of Pediatrics, 2000.
American Academy of Pediatrics. Committee on Infectious Diseases. "Recommended Childhood Immunization Schedule: United States, January-December 2001.'' Pediatrics 107, no. 1 (2001):202-204.
Centers for Disease Control and Prevention. "Achievements in Public Health, 1900-1999: Impact of Vaccines Universally Recommended for Children—United States, 1990-1998.''
Morbidity and Mortality Weekly Report 48, no. 12 (1999):243-248.
Centers for Disease Control and Prevention [web site]. Atlanta, GA, 2001. Available from http://www.cdc.gov; INTERNET.
Dales, L., S. J. Hammer, and N. J. Smith. "Time Trends in Autism and in MMR Immunization Coverage in California.'' Journal of the American Medical Association 285, no. 9 (2000):1183-1185.
Radetsky, Michael. "Smallpox: A History of Its Rise and Fall.'' Pediatric Infectious Disease Journal 18, no. 2 (1999):85-93.
Taylor, Brent, Elizabeth Miller, C. Paddy Farrington, MariaChristina Petropoulos, Isabelle Favot-Mayaud, Jun Li, and Pauline A. Waight. "Autism and Measles, Mumps, and Rubella Vaccine: No Epidemiologic Evidence for a Causal Association.'' Lancet 353 (June 12, 1999):2026-2029.
Wakefield, A.J., S. H. Murch, A. Anthony, J. Linnell, D. M. Casson, M. Malik, M. Berelowitz, A. P. Dhillon, M. A. Thomson, P. Harvey, A. Valentine, S. E. Davies, and J. A. Walker-Smith. ''Ileal-Lymphoid-Nodular Hyperplasia, Non-specific Colitis, and Pervasive Developmental Disorder in Children.'' Lancet 351 (February 28, 1998):637-641.
In vitro fertilization is the term for a process whereby a mature egg from the female and a sperm from the male are placed in culture media where fertilization can occur. For humans, the first clinically successful in vitro fertilization occurred in 1978. If accomplished, cell division results in six to eight cells in about forty-eight hours, or a blastocyst of 100 cells in about 120 hours. One or more can then be transferred into the uterus with a 20 percent to 60 percent expectation of pregnancy depending on many variables, including age, cause of infertility, and number of fertilized eggs, or pre-embryos, transferred.
Pregnancy rates increase with number of pre-embryos transferred, as do the multiple pregnancy rates. In the United States (1998), 360 clinics conducted 80,634 treatment cycles; 31 percent of deliver ies were multiple, compared to 3 percent in the general population.
In vitro fertilization has expanded to include the use of donor eggs, donor sperm, cryopreservation, in-tracytoplasmic sperm injection (ICSI), and the use of surrogate uteri.
See also: ARTIFICIAL INSEMINATION; REPRODUCTIVE TECHNOLOGIES
Rabe, Thomas, Klaus Diedrich, and T. Strowitzki. Manual on Assisted Reproduction. Berlin: Springer-Verlag, 2000.
cial support for the Parts B and C programs. Nationwide over six million eligible children receive a free appropriate public education (FAPE), and almost 200,000 infants and toddlers are served. The U.S. Department to Education and its Office of Special Education and Rehabilitative Services administers this $7.4 billion program that aims to produce positive results for children across the nation.
See also: DEVELOPMENTAL DISABILITIES
Council for Exceptional Children (CEC) for the IDEA Partnership Projects. Available from http://www.ideapractices.org/ lawandregs.htm; INTERNET.
Pascal Louis Trohanis
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Autism is a developmental disorder that manifests itself in early childhood and affects the functioning of the brain, primarily in the areas of social interaction and communication. Children with autism look like other children but do not play or behave like other children. They must struggle daily to cope and connect with the world around them.