My son is an only, because infertility. I'm an only, because recurrent miscarriage. My father is an only, because WW2. [And he's kept a close bond with childhood friends through 8 decades, though they scattered over the country/world, whereas my mum and in-laws have distant cards-at-Christmas relationships with their siblings, so take that, only child stereotypies!]
The Victorian psychologist and gentleman G Stanley Hall gave us the spoiled only child stereotype; he declared being an only child to be a "disease" in itself and that singletons were deficient, indulged, spoiled. (He also wrote extensively about women's role as the Angel of the House, declared intellectual women a "biological deformity" and advocated a process called "retarding" the education of girls to repress any curiosity about science, history or politics. Super.)
His ideas about onlies have been widely debunked in academia but are still one of those "truths universally acknowledged" in society.
For the academic debunking, see eg "Only Children and Personality Development: A Quantitative Review" by Polit and Falbo, Journal of Marriage and Family Vol. 49, No. 2 (May, 1987), pp. 309-325. "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."
In other words, people is people, in all their rich variety, whether you've got siblings or not.