The research involving the psychological effects of adoption on children is, I think, inconclusive at best, contrary at worst. Often times the long term reaction or the development of someone who is adopted is related to many other factors in combination, including the economic level of the adopting family, family stability, whether or not there is still contact with the natural parents, and the overall personality and intelligence of the child themselves. Sometimes deep-seeded feelings of abandonment take place in adopted children, while others rarely give their birth parents a second thought.
I agree with other editors in the way that there are no hard and fast answers. There seem to be adopted children who grow up to be very secure about themselves and their identity, and then there are also adopted children, that we seem to hear a lot more about, who have real struggles and issues regarding their identity formation. I would tend to favour the influence of nurture as being one instrumental factor, but, as always when we consider human beigns, we need to remember that we are not neat mathematical formula that will always produce the same, predictable answer.
A lot of research is currently being done on adoption. In one area, research shows that infants who are adopted are also vulnerable to a sense of "loss and a search for self." The irony is that in other cultures and earlier eras, being adopted was seen as an honor since the adopted person was chosen. I wonder what has changed or if history has simply provided inadequate information.
My brother is leaving tomorrow to adopt three siblings from Brazil, and we have been intensely listening to and reading about adoption, as well as talking to adoptees, for the past year, in particular. What I have gathered is that secrets are no good (children should be told they are adopted), that children should be allowed to remember and talk about the parents they lost (in the case of orphans or foster children), and that there will be unknown "triggers" which children will have based on their pre-adoption experiences. Adoption is a wonderful thing, but especially in the case of older children there are likely to be far-reaching implications--not all of which are negative, of course.
I have not had any experience with this other than what I've read or seen on TV, but it seems to me that the experiences would be vast and varied. For instance, if the adopted child's adopted parents were loving and nurturing people, the child would be very well-adjusted and possibly never even think about his/her biological parents. On the other hand, because the child is so well-adjusted, his or her natural curiosity may get the better of him/her and the desire to meet the biological parents may be overwhelming, just to get a little closure and a sense of what may have been missed or why he/she was given up for adoption in the first place.
If the child is not well-adjusted or grew up in the foster care system, there may be an immense sense of injustice, loss, anger, and even a desire for revenge. Many children in the foster care system are abused and mistreated which would manifest these negative feelings and perhaps project them onto parents who "threw them away".
There is so much variety in experiences of adopted children that it's hard to answer this question. Sometimes they do feel a sense of loss from not having known their real parents. Adopted adults have been known to seek out birth parents, which is not always a happy prospect, just to get closure.
While I am not familiar with the research aspect of this topic, I firmly believe in nature v. nurture. I must agree with post #2, I think "is that the psychological outcome of adopted children probably varies as much as the psychological outcome of non-adopted children." Children are the product of the environment of which they are raised. If the environment is a loving one, the child will probably not have the psychological issues of a child not raised in a loving home. (Remember, this goes both ways- adopted and natural children.)
When I googled research on the topic, the main suggestions tied to children and psychological issues were labeled with "divorce", "trauma", and "soldier's children". Not one search immediately offered an article on adopted children.
Good luck with your research.
Perhaps this should be moved to the Discussion Forum - I'm sure you would receive a variety of anecdotal responses based on personal experiences or observations. My personal reaction, based on watching my Chinese-American niece for the past almost ten years, is that the psychological outcome of adopted children probably varies as much as the psychological outcome of non-adopted children - and for the same variety of reasons.
You don't specify what you are considering "later in life." As with most children, possibly the most intense period of psychological growth and potential difficulties may be the adolescent years.
...adolescence is seen as a time of identity formation and emerging independence. Adopted adolescents are faced with the challenge of integrating disparate sources of identity— their biological origins and their family of rearing—as they establish themselves as individuals.
However, adolescents who are not adopted are faced with the same challenge and may also struggle with finding their separate identity. The way in which parents and families handle the history and explanation of the adoption is the critical factor.