A group of researchers reviewed numerous studies that explored the relationship between sugar intake and weight outcomes (weight loss, weight gain, BMI, etc).  The citation for the study is BMJ 2013;346:e7492  and you can read it in full by clicking here.  I think the meta analysis was very well done.  The scientists used strict criteria in deciding which studies to include and also in interpreting the findings from those selected.
    Studies included adults and children and were mostly quasi-experimental and/or involved cohort and correlation studies.  Because there were no studies that used a treatment and control group to compare sugar users to non sugar users while holding everything else about people the same, a conclusion that sugar 'causes' weight gain can not be claimed.  
   However,  the research does indicate that the people who consume the most sugar weigh the most.  There is an association between sugar intake and weight.  It is likely that people who consume the most sugar also consume the most calories.  One of the studies that was reviewed included a group of people who ate the same amount of calories, but replaced the sugary foods with other carbohydrates and in that study there was no difference between the two groups weights (or weight gain/loss)
   I am really simplifying things here.  The bottom line was that sugar intake and weight were related, but it may just be that too many calories is too many calories.  That is different from some  research that indicates a different metabolic response to calories from sugar.  Stay tuned for more as research into this matter continues. 
   I hope you will check out the study that I linked above, here is the summary paragraph regarding the conclusion the authors made:
Among free living people involving ad libitum diets, intake of free sugars or sugar sweetened beverages is a determinant of body weight. The change in body fatness that occurs with modifying intakes seems to be mediated via changes in energy intakes, since isoenergetic exchange of sugars with other carbohydrates was not associated with weight change. 
    To break break that down.  Free living means people in their every day lives (environments/context) and ad libitum diets means eating whatever is usual ( no diet restrictions, etc) and isoenergetic means the same amount of calories.  So regular people like me and you who eat a lot of sugary foods or drink sugary drinks will weigh more than people who do not - but this seems to be due to the fact that a high sugar diet is also high in calories.