A maioria das pessoas daqui cresceram levando palmadas na bunda. 100% dos meus amigos de infância cresceram levando palmadas na bunda. Os estudos não mostram uma ligação direta entre "punição física" e efeitos negativos a longo prazo nas crianças.
Não, é um pouco como cigarro e câncer. Muita gente fuma e não desenvolve câncer de pulmão ou outros ligados ao tabaco, chuto que provavelmente você não conhece um caso pessoalmente, apesar de conhecer muitos fumantes. No entanto existe sim a relação causal, apenas não é 100%.
O texto coincide de lidar com o contraponto do primeiro texto que eu citei:
MORE HARM THAN GOOD: A SUMMARY 
OF SCIENTIFIC RESEARCH ON THE 
INTENDED AND UNINTENDED EFFECTS 
OF CORPORAL PUNISHMENT ON 
CHILDREN 
[...]
The bulk of the criticism of the empirical research on corporal punishment 
comes from two researchers, Diana Baumrind and Robert Larzelere.120
 
Although these two authors are both prolific and vociferous, their opinions 
should not be mistaken for the views of the mainstream researchers in the fields 
of psychology, medicine, or education. They find fault with the research 
showing negative outcomes of corporal punishment and point to studies that fail 
to find statistically significant negative outcomes. But they are equally unable to 
cite a body of research showing positive long-term outcomes of corporal 
punishment, for such a body of research does not exist. Despite the lack of 
empirical evidence for their position, these authors criticized the Gershoff 
meta-analysis cited above and concluded that, even though negative outcomes 
were associated with corporal punishment in ninety-four percent of the studies, 
the research to date did “not justify a blanket injunction against mild to 
moderate disciplinary spanking.”121 Elsewhere, Larzelere has argued that the 
research cannot be trusted because it is based primarily on correlational data.122 
Such an assertion indicates that the author will never be convinced by the 
available data because it is impossible to study parents’ use of everyday 
spanking in an experimental fashion. Although randomized studies with 
treatment and control groups are the “gold standard” of the basic and medical
sciences, children cannot be randomly assigned to parents in experimental 
designs,123 nor, since the 1980s, have institutional review boards approved 
studies that randomly assign parents to spank or not spank their own children.124
 
However, parenting researchers work diligently to make up for the lack of 
experimental designs by creating carefully selected and representative samples 
of families and by employing a range of new statistical methods that make 
better estimates of causal parameters from observational data.125
 
Well-designed correlational research has led to several public-health 
conclusions and intervention efforts over the last few decades. To take but one 
example, the now well-accepted fact that cigarette smoke causes lung cancer is 
based on a body of correlational research.126 Clearly, it would be unethical to 
randomly assign people to smoke or not, so researchers must instead rely on 
longitudinal but correlational studies that follow individuals who themselves 
choose to smoke. These studies attempt to take into account as many factors as 
possible that may account for who smokes and who does not in the first place.127
 
This body of correlational research does not meet the high bar set by Baumrind 
and Larzelere, but it nonetheless led the U.S. Surgeon General to conclude that 
the “evidence is sufficient to infer a causal relationship” between smoking and 
cancers of the bladder, blood (leukemia), cervix, esophagus, kidneys, larynx, 
lungs, mouth, pancreas, and stomach, among many other serious health 
consequences.128 This research does not suggest that smoking one cigarette will 
cause an individual to develop cancer, but rather that the risk increases with 
each cigarette smoked and, conversely, that if an individual never smoked, his 
or her risk for these negative health outcomes is greatly reduced. Similarly, the 
research to date does not support a conclusion that one spank will cause a child 
to become aggressive or delinquent; rather, with every spank the risk 
increases.129 Never spanking at all would provide the lowest risk for such 
negative outcomes.
[...]
http://scholarship.law.duke.edu/cgi/viewcontent.cgi?article=1565&context=lcp
O que vocês estão tentando fazer? Redesenhar a realidade e apontar severos problemas psicológicos em gerações de pessoas que cresceram levando tapas na bundinha?
Não, da mesma forma que não se sugere que todo mundo que fuma esteja caquético, mas pode-se dizer que fumar é prejudicial à saúde.
Ao mesmo tempo, é provável que essas gerações tivessem se desenvolvido melhor, se tivessem sido criadas de outra maneira. Não é como se todas as gerações anteriores/atuais fossem 100% saudáveis e se achasse que isso só fosse valer agora. 
E veja bem, eu não estou defendendo abuso infantil. Não estou defendendo tapa na cara, surra de cinto, chute na barriga, voadora nas costas, etc. Estou defendendo que tapas na bundinha, que possuem um efeito muito mais psicológico do que físico, são, em certas situações, mais efetivos do que apenas um xingamento, castigo, etc.
Acho que xingamento também não é algo adequado*.
E sim, o efeito é psicológico mesmo, não é só lesão física o problema.
Não existe na literatura científica recomendações de um grau de severidade "ideal" para castigos físicos a crianças.
* não surpreendentemente:
Verbal aggression by parents and psychosocial problems of children ☆
Yvonne M. Vissing, Murray A. StrausCorresponding author contact information
Family Research Laboratory, University of New Hampshire, Durham, NH, USA
Richard J. Gelles
University of Rhode Island, Kingston, RI, USA
John W. Harrop
Brown University Medical School, USA
Abstract
Analyses of data on a nationally representative sample of 3,346 American parents with a child under 18 living at home found that 63% reported one or more instances of verbal aggression, such as swearing and insulting the child. Children who experienced frequent verbal aggression from parents (as measured by the Conflict Tactic Scales) exhibited higher rates of physical aggression, delinquency, and interpersonal problems than other children. This relationship is robust since it applies to preschool-, elementary school-, and high school-age children, to both boys and girls, and to children who were also physically punished as well as those who were not. Children who experienced both verbal aggression and severe physical violence exhibited the highest rates of aggression, delinquency, and interpersonal problems.