I'm wondering what real, concrete evidence there is one way or the other? Is a child really better off with a sibling?
How do you measure this? "Better off" is potentially subjective, and only-child vs. having siblings is likely a trade-off of some positives and some negatives for each side.
Studies appear to be somewhat contradictory, likely because of the subjective nature.
Susan Newmann, a Social Psychologist, advocates for only children being no different, on the whole, from children with siblings.
This opinion piece cites numerous studies (unfortunately most are not specifically identified) that indicate there are no significant differences between only-children and those with siblings:
in hundreds of studies during the past decades exploring 16 character traits — including leadership, maturity, extroversion, social participation, popularity, generosity, cooperativeness, flexibility, emotional stability, contentment — only children scored just as well as children with siblings.
An Ohio State survey of more than 13,000 children found that only children had as many friends as anyone else
Some studies even suggest that only children may have some advantages over children with siblings:
This review combined the results of 141 studies and found that only children scored significantly better than other groups in achievement motivation and personal adjustment. The achievement motivation finding was especially reliable, persisting across several comparison groups. Overall, however, the review indicated that only children were comparable in most respects to their siblinged counterparts.
However, on the other side of the coin, studies were done to evaluate the differences between children born before and after the "One Child" policy introduced in China in 1979.
The team had the participants play simple economics games, which involve the exchange of money between anonymous participants and are designed to test a range of personality indicators. The games revealed that people born after the introduction of the policy were, not only less trusting, less trustworthy, and more pessimistic, but also less competitive, less conscientious, and more risk-averse.
While the article I link extolls the scientific virtues of this study, personally I am concerned that they're identifying the relationship as causal, even though the children are of different generations.
Sometimes children might feel that their parents favor their sibling(s) over them. Research on favoritism shows this can have some negative consequences:
Self-esteem in the child correlates to perceived parental favoritism (Felson & Zielinski, 1989; Zervas & Sherman, 1994). Children, whose parents had treated all children equally, rated highest in self-esteem; favored children rated lower, nonfavored lowest (Zervas & Sherman, 1994). Parents' supportive behavior affects the self-esteem of children but child self-esteem also affects how much support children report their parents give them; in addition, parents have a greater effect on the self-esteem of girls than of boys (Felson & Zielinski, 1989).
Siblings express less warmth and greater hostility toward one another when parents show favoritism (McHale et al., 1995)
over time, being unfavored by parents produces behavior problems in children (Richmond, Stocker, & Rienks, 2005)
I am an only child, as is my cousin, who is the family member I've always had the closest relationship with. We've discussed this topic quite a bit between us.
We both agreed that being an only child had disadvantages. We both felt that not having siblings took away opportunities from us. Opportunities to be or have role models close to our age. Opportunities to have readily-available playmates at home.
We've also discussed other benefits, such as saving money on hand-me-downs, and the kids entertaining themselves more readily, freeing up some "me" or "us" time for the parents.
Some of the negatives I listed above can also be viewed as positives.
Lack of readily-available playmates (I grew up in a suburban neighborhood with very few children; most of my neighbors were older, with grandchildren) made me adept at finding ways to amuse myself (including developing a love of reading).
I believe that my being an only child has generally made me more independant
There are also potential downsides for having siblings.
Hand-me-downs are an economic advantage for the parents, but may be problematic for the ones receiving them.
And ultimately, some siblings just do not get along.
My wife is one of three daughters. Her relationship with one of her sisters is, and always has been... well, "strained" is the most positive term I can use to describe it. Much of their history has been outright hostile.
I know other people who grew up resentful because they felt that they were shouldered with the burden of being responsible for their younger sibling(s). Of course, others had the exact opposite perception.
Given the mix of pros and cons for each, ultimately it should be what works best for the parents that should be the deciding factor.
My cousin wound up having two children, for exactly the reasons I mentioned above. She is very happy that they have grown to be very good friends. Sure, there is occasional strife, but generally they stick up for each other, play well with each other, and share a strong bond. It does seem to give my cousin and her husband more time with the kids off doing their own thing, but I suspect that that benefit may be short-lived, especially if the kids start getting involved in extracurricular activities (one is school-aged, the other will be soon).
My wife and I, on the other hand, wound up deciding that we're sticking with one child, despite my previous belief that I wanted two. There were a number of factors in our decision, ranging from space (our house isn't terribly large, so room-sharing would be a potential issue) to finance (kids, especially infants, are expensive!) to time (newborns are a tremendous investment in time as well as money, and we don't want to have to start off with diapers all over again!).
Ultimately, though, I think what sealed the deal for us was just how happy and well-adjusted our son appears, even though he doesn't have a sibling. He plays very well with others, has always shared exceptionally well for his age, and has a number of friends that he's made, in daycare and out.
In short, do what you think is best, and ignore the opinion of others. After all, if you do wind up having another child, eventually you'll probably start hearing people commenting on how much better only children have it!