We begin this special section with an article entitled “Toward a Research Agenda.” To some, this subtitle may seem strange. After all, Grossman (1972) published her landmark study of college-aged siblings more than 3 decades ago. Since that time, numerous investigators have examined sibling feelings, functioning, and relationships. Meyer and Vadasy (1994) have produced “sibshops” to help siblings, and both popular (McHugh, 2003) and scholarly (Stoneman & Berman, 1993) books have appeared in which authors described issues involved in growing up as a sibling of a brother or sister with disabilities.

Despite such advances, however, research on siblings of individuals with disabilities remains underdeveloped. As Stoneman (2005) noted, many researchers have produced one or two sibling studies, but, except for some of the contributors to this special issue, few investigators are primarily identified as “sibling researchers.” Sibling studies are also beset with difficult methodological, statistical, and practical issues, and many topics have not been examined.

Ironically, this underdeveloped field exists within the context of demographic changes that highlight sibling relationships. Consider the issue of aging. Like the overall general population, persons with disabilities are living increasingly longer lives. Currently, 526,000 Americans with disabilities are 60 years or older and that number is expected to triple—to over 1.5 million—by 2030 (National Center for Family Support, 2000). Given that 60% of these adults are cared for by aging parents in the family home, who will take care of them when their parents no longer can? Adult siblings of adults with disabilities are the assumed future caregivers, but the needs of adult siblings have remained relatively unexplored.

Faced with this discrepancy between an underdeveloped field and an important social issue, we decided to edit a special section of Mental Retardation devoted to siblings of persons with disabilities. Our goal is to stimulate additional research by identifying themes that highlight current challenges and future directions. To that aim, we described six themes in this article.

Theme 1: Methodological Challenges

Like any underdeveloped area, the study of siblings is fraught with methodological challenges. Specifically, in future studies we need to do several things:

Advance Beyond Small Samples of Convenience

Like many areas of family research in mental retardation, most investigators conducting sibling studies examine small numbers of participants— usually from 20 to 50 per group. Although small-sample studies are common in family research, any two siblings feature special demographic characteristics. Compared to the child with disabilities, individual siblings can be younger or older, of the same or different gender, and widely or narrowly separated in age. In addition, siblings can come from larger or smaller families, of varying socioeconomic (SES) levels, and ethnicities. In small-scale studies, researchers have difficulty in teasing apart these more subtle and interacting variables.

An additional complication concerns the source for samples in sibling studies. In most studies, siblings are volunteers recruited from specialized clinics–hospitals or parent groups. Demographic characteristics of persons served by particular clinics or hospitals, however, often change over time, and those families who become involved in parent and advocacy groups may differ from less involved families (Rowitz, 1993). Although occasionally in family studies, researchers have utilized larger, epidemiological databases (e.g., Seltzer & Greenberg's, 2001, use of the Wisconsin Longitudinal Study—most studies are limited in size and potentially unrepresentative. Similar to those in other areas, sibling researchers have been guilty of “taking what they can get” in terms of research participants, and the effects of adopting such a research strategy remain unclear.

Consider Which Siblings to Include as Research Participants

Families often have more than one nondisabled sibling. Particularly in research on adult siblings, most investigators examine the sibling who has the most contact or a closer emotional relationship to the brother or sister with disabilities. Such studies have informed us about the “gendered” nature of adult–sibling caretaking, of how sisters as opposed to brothers are generally more involved in the care of the adult brother or sister with mental retardation (Orsmond & Seltzer, 2000).

To what extent do such findings generalize to other siblings in the family? How might less-involved siblings feel and what are these siblings' relationships with their brother or sister with disability? How, when, and why do such family patterns become established? Several methodological approaches might be needed to expand our study of sibling relationships over the life-span, including: (a) studying more than one sibling of the individual with disability; (b) systematically sampling older and younger siblings; and (c) analyzing the contributions of age span, gender, and size of sibling group on both childhood and later life outcomes.

Better Understand Control–Contrast Groups

To date, sibling researchers have paid insufficient attention to control or contrast groups. Many investigators have no control or contrast group, others compare siblings of persons with disabilities to those without, still others use contrast groups with different disabilities (e.g., siblings of children with Down syndrome versus those with autism).

The important issue concerns what one wants to know. So far, most researchers have asked whether siblings of children with disabilities differ from siblings of typically developing children, but other questions are also pertinent. Using a strategy suggested for family studies of children with autism, one might examine the effects of different child characteristics by employing more than one control or contrast group (Seltzer, Abbeduto, Krauss, Greenberg, & Swe, 2004). Essentially, different control–contrast groups serve as proxies for children who present their families with specific problems and experiences (Hodapp, 2004). Also unresolved is the degree to which persons with disabilities demonstrating a particular behavior are reacted to differently when they also have a specific genetic diagnosis (e.g., Down syndrome).

Unyoke Shared Genes From Shared Environments

Siblings with the same birth parent share half of their genes and a similar family environment. Statistically and methodologically, how can one disentangle genetic from environmental influences? Such issues become complicated in that for complex traits, such as intelligence, familial genetic influences become more—not less—pronounced as children get older (Plomin & Spinath, 2004). In addition, genetic loading has been identified in such disorders as autism, with various family members “on the spectrum,” demonstrating lesser versions of their brother or sister's problems (Bolton et al., 1994). Finally, in fragile X syndrome, parents are carriers of their child's genetic disorder and may themselves show heretofore unappreciated medical or psychiatric problems (Hagerman & Hagerman, 2004).

Despite such complications, siblings of a brother or sister with disabilities may present unique opportunities to untangle genetic from environmental effects. To disentangle these factors, sibling researchers will need to borrow from behavior geneticists the necessary methodologies and statistical tools and more judiciously use control or contrast groups. We can, however, learn much from the natural experiment of siblings of individuals with different disabilities.

Theme 2: Measurement

At present, few good measures exist to evaluate sibling experiences and relationships when one offspring has developmental disabilities. Part of the issue involves the dual relationship when one sibling has disabilities. On one hand, the usual feelings and perceptions measured in all sibling studies remain relevant. Like all sibling pairs, siblings and their brother or sister with disabilities vary on warmth/ closeness, status/power, companionship, affection, nurturance, admiration of and by the sibling, and conflict/rivalry (Forman & Burhmester, 1985). On the other hand, other feelings or experiences may be unique to siblings of a person with disabilities. Issues of shame, pride, over-protectiveness, extra caregiving—even of potentially living a fuller, more nuanced life—all relate more to siblings of individuals with disabilities (McMillan, 2005; Swenson, 2005). Such issues are rarely tapped by the most commonly used sibling measures.

In addition, little work has yet been conducted on the varying levels of sibling measurement. Thus, one could examine how the experience of growing up as a sibling to a brother or sister with disabilities has either helped or hindered that sibling's life or affected the sibling's degree of empathy or life views more generally. Conversely, investigators could examine whether (and which variables predict that) siblings of a brother or sister with disabilities are more likely to show increased levels of psychological stress, greater numbers or more severe types of illnesses, or other problems. One might ask as well about the sibling relationship, how each member (the offspring with and without disability) conceptualizes the relationship, and how that relationship is similar to or different from other sibling relationships within the family or from sibling pairs in families who do not have an offspring with disabilities.

In addition, although some researchers have examined sibling–sibling interactions (see Stoneman, 2001), most focus on the self-reported feelings of the sibling without disabilities. In future work, researchers need to rely on a wider variety of informants, including mothers, fathers, other children in the family, the brother or sister with disabilities, teachers, and friends. Researchers also need to measure siblings' self- and other-reported adjustment, school and job performance, health, and the ability to engage in marital and parental relationships. If our questions concern the overall functioning of these siblings, then multiple outcome measures seem necessary.

Issue 3: Developmental and Life-Course Perspectives

A third major challenge involves studying sibling development over time, even into the adult years. When one of the siblings has disabilities, few researchers have examined such age-related changes. Such inattention has occurred even though anecdotal (McHugh, 2003) and clinical (Meyer & Vadasy, 1994) evidence suggests that reactions to the brother or sister with disability may differ based on the age of the sibling.

A related issue concerns a life-span perspective. Although recent issues of the American Journal on Mental Retardation and other journals have examined life-span development, few articles have appeared concerning siblings. We are, therefore, left with many unanswered questions about life-span development of siblings and sibling relationships. Are there particular times during development— transition points for either the sibling or for the brother or sister with disabilities—that are more difficult? Which variables predict which siblings will be more versus less likely to successfully assume caretaking roles during adulthood? How does the nature, amount, and timing of service-delivery intersect with sibling relations during adulthood? Developmental issues of adult siblings remain mostly unexamined, even as the public policy challenges intensify.

Issue 4: Mediators–Moderators

In the fields of family studies, developmental psychopathology, and developmental psychology, increasing attention is being paid to mediators and moderators. Researchers also need to attend to these issues in sibling research.

Mediators

Mediators involve the degree to which the pathway from Variable A to Variable C “goes through” Variable B. To give one example, it is now fairly well-established that mothers who adopt more “active” styles of problem-solving are less likely to be depressed and more likely to cope well with raising a child who has disabilities. To what extent are siblings' reactions to their brother or sister with disabilities mediated by their mother's coping style? Are there other mediational variables relating to parents, families, or services received?

Moderators

Moderators involve the degree to which an effect is present only when another specific variable is present or absent. For example, different relationships may hold when the target parent of the child with disabilities is a mother versus a father (Hastings & Brown, 2002). In sibling studies, the key role of moderators—based on the gender of the sibling or of the brother versus sister with disabilities, ethnicity, or rural–urban residences—are all poorly understood. We also know little about the effects on sibling outcomes of different family structures (relating to birth order, age-spacing, number of children in the family) or different family dynamics (e.g., the way in which some families are open versus closed in their communication styles).

Larger scale, multi-site studies seem necessary. Other promising approaches involve epidemiological examinations of administrative records (e.g., the use of family-linked administrative data to examine the health and medical histories of siblings).

Issue 5: Cultural and Subcultural Issues

To date, most family studies in disabilities have focused primarily on Euro American, middle-class families. Use of only this group also characterizes studies of siblings with disabilities, although this state of affairs is gradually changing.

Within the sibling context, one sees important cultural and subcultural issues. In many societies, siblings are the primary caregivers of children (Weisner & Gallimore, 1977). The sibling role is probably less valued in American culture, though it remains important. In addition, many differences exist across subcultures within American society. For example, the percentages of adults with disabilities living in family homes (versus in independent apartments or in group homes) is higher in Latino versus Anglo populations, and much discussion in family circles concerns the role in Latino groups of familism, or strong family identification and ties (Blacher, 2001). Yet, how such family-related feelings and perceptions relate to sibling perceptions, actions, or understandings have yet to be adequately examined.

Issue 6: Balanced Views: Both Positive and Negative

A final issue concerns a balanced portrayal. Across several decades, family research has been accurately described as focusing more on negative than on positive outcomes. Even now, some have argued that disability family research is more accurately characterized as “less negative” in orientation rather than focusing on positive aspects per se (Helff & Glidden, 1998).

Again, trends in the larger field of psychology can aid sibling researchers. Peterson and Seligman (2004) have recently proposed a system of character strengths. These include creativity, open-mindedness, love of learning, kindness, love, fairness, leadership, humility, hope, spirituality, and gratitude. Although one can debate the exact nature and number of separate strengths, positive personality characteristics clearly exist and are subject to change based on life experiences.

If personal strengths are malleable based on one's life experiences, will siblings of persons with disabilities show increased levels of specific personal strengths? As Dykens (2005) noted, several strengths are likely elevated in siblings of persons with disabilities, though positive outcomes receive scant attention in most sibling studies (Glidden & Schoolcraft, in press). Even in Grossman's (1972) original study, roughly half of the siblings described feeling more open, tolerant of differences, and focused on life's larger meanings as a result of growing up with a sibling who has disabilities.

We are left, then, with a burgeoning field, with hints from yesterday's studies of tomorrow's findings. Granted, sibling researchers need to pay greater attention to methods, measures, developmental and life-course perspectives, mediators–moderators, culture, and more balanced views. At the same time, however, critical underpinnings for a new era of sibling research already exist. We hope that this article—and the other articles in this special section— foster a fuller, more elaborated research agenda concerning siblings of persons with disabilities.

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Author notes

Authors: Robert M. Hodapp, PhD (robert.hodapp@vanderbilt.edu), Professor, Department. of Special Education, and Co-Director, Family Research Program, and Ann Kaiser PhD, Professor, Special Education, Vanderbilt Kennedy Center, Peabody Box 328, 230 Appleton Place, Nashville, TN 37203. Laraine Masters Glidden, PhD, Professor, Department of Special Education, St. Mary's College of Maryland, 18952 E. Fisher Rd., St. Mary's City, MD 20686