Humans generally consider humans to be rather intelligent, at least by human standards. This is primarily because the standard is set by humans.

Looking at this, we can see that computers were designed by our human intelligence. All computers that are manufactured and marketed by a company run by humans tend to process data in exactly the same fashion. This enables us to create programs that work the same on all systems.

Looking at humans, however, we find that all humans do not process information the same way. Many cannot even follow simple directions on the highway. There is a great deal of diversity in the way that humans think about things, and many different ways of interpreting data.

Thus it can be established that if humans are "intelligently designed", then they must be designed by many competing intelligences.

However, then comes the problem of siblings. Any two or more siblings evidently must come from the same manufacturer. Yet it is quite rare to find siblings, even identical twin siblings, who process data with any degree of similarity.

Thus, the intelligent designers must be either extremely inept, or humans just aren't intelligently designed.